Improved string diff viewer (#994)

CHANGELOG.md

@@ -31,8 +31,10 @@ Making a new release? Simply add the new header with the version and date undern
 
 ## Unreleased
 
+* Fixed bugs and improved performance & UX for the script diff viewer ([#994])
 * Added support for `.jsonc` files for all JSON-related files (e.g. `.project.jsonc` and `.meta.jsonc`) to accompany JSONC support ([#1159])
 
+[#994]: https://github.com/rojo-rbx/rojo/pull/994
 [#1159]: https://github.com/rojo-rbx/rojo/pull/1159
 
 ## [7.6.1] (November 6th, 2025)

plugin/src/App/Components/CodeLabel.lua

@@ -1,66 +0,0 @@
-local Rojo = script:FindFirstAncestor("Rojo")
-local Plugin = Rojo.Plugin
-local Packages = Rojo.Packages
-
-local Roact = require(Packages.Roact)
-local Highlighter = require(Packages.Highlighter)
-Highlighter.matchStudioSettings()
-
-local e = Roact.createElement
-
-local Theme = require(Plugin.App.Theme)
-
-local CodeLabel = Roact.PureComponent:extend("CodeLabel")
-
-function CodeLabel:init()
-	self.labelRef = Roact.createRef()
-	self.highlightsRef = Roact.createRef()
-end
-
-function CodeLabel:didMount()
-	Highlighter.highlight({
-		textObject = self.labelRef:getValue(),
-	})
-	self:updateHighlights()
-end
-
-function CodeLabel:didUpdate()
-	self:updateHighlights()
-end
-
-function CodeLabel:updateHighlights()
-	local highlights = self.highlightsRef:getValue()
-	if not highlights then
-		return
-	end
-
-	for _, lineLabel in highlights:GetChildren() do
-		local lineNum = tonumber(string.match(lineLabel.Name, "%d+") or "0")
-		lineLabel.BackgroundColor3 = self.props.lineBackground
-		lineLabel.BorderSizePixel = 0
-		lineLabel.BackgroundTransparency = if self.props.markedLines[lineNum] then 0.25 else 1
-	end
-end
-
-function CodeLabel:render()
-	return Theme.with(function(theme)
-		return e("TextLabel", {
-			Size = self.props.size,
-			Position = self.props.position,
-			Text = self.props.text,
-			BackgroundTransparency = 1,
-			FontFace = theme.Font.Code,
-			TextSize = theme.TextSize.Code,
-			TextXAlignment = Enum.TextXAlignment.Left,
-			TextYAlignment = Enum.TextYAlignment.Top,
-			TextColor3 = Color3.fromRGB(255, 255, 255),
-			[Roact.Ref] = self.labelRef,
-		}, {
-			SyntaxHighlights = e("Folder", {
-				[Roact.Ref] = self.highlightsRef,
-			}),
-		})
-	end)
-end
-
-return CodeLabel

plugin/src/App/Components/StringDiffVisualizer/StringDiff.lua

@@ -1,3 +1,4 @@
+--!strict
 --[[
     Based on DiffMatchPatch by Neil Fraser.
     https://github.com/google/diff-match-patch
@@ -67,8 +68,187 @@ function StringDiff.findDiffs(text1: string, text2: string): Diffs
 	end
 
 	-- Cleanup the diff
+	diffs = StringDiff._cleanupSemantic(diffs)
 	diffs = StringDiff._reorderAndMerge(diffs)
 
+	-- Remove any empty diffs
+	local cursor = 1
+	while cursor and diffs[cursor] do
+		if diffs[cursor].value == "" then
+			table.remove(diffs, cursor)
+		else
+			cursor += 1
+		end
+	end
+
+	return diffs
+end
+
+function StringDiff._computeDiff(text1: string, text2: string): Diffs
+	-- Assumes that the prefix and suffix have already been trimmed off
+	-- and shortcut returns have been made so these texts must be different
+
+	local text1Length, text2Length = #text1, #text2
+
+	if text1Length == 0 then
+		-- It's simply inserting all of text2 into text1
+		return { { actionType = StringDiff.ActionTypes.Insert, value = text2 } }
+	end
+
+	if text2Length == 0 then
+		-- It's simply deleting all of text1
+		return { { actionType = StringDiff.ActionTypes.Delete, value = text1 } }
+	end
+
+	local longText = if text1Length > text2Length then text1 else text2
+	local shortText = if text1Length > text2Length then text2 else text1
+	local shortTextLength = #shortText
+
+	-- Shortcut if the shorter string exists entirely inside the longer one
+	local indexOf = if shortTextLength == 0 then nil else string.find(longText, shortText, 1, true)
+	if indexOf ~= nil then
+		local diffs = {
+			{ actionType = StringDiff.ActionTypes.Insert, value = string.sub(longText, 1, indexOf - 1) },
+			{ actionType = StringDiff.ActionTypes.Equal, value = shortText },
+			{ actionType = StringDiff.ActionTypes.Insert, value = string.sub(longText, indexOf + shortTextLength) },
+		}
+		-- Swap insertions for deletions if diff is reversed
+		if text1Length > text2Length then
+			diffs[1].actionType, diffs[3].actionType = StringDiff.ActionTypes.Delete, StringDiff.ActionTypes.Delete
+		end
+		return diffs
+	end
+
+	if shortTextLength == 1 then
+		-- Single character string
+		-- After the previous shortcut, the character can't be an equality
+		return {
+			{ actionType = StringDiff.ActionTypes.Delete, value = text1 },
+			{ actionType = StringDiff.ActionTypes.Insert, value = text2 },
+		}
+	end
+
+	return StringDiff._bisect(text1, text2)
+end
+
+function StringDiff._cleanupSemantic(diffs: Diffs): Diffs
+	-- Reduce the number of edits by eliminating semantically trivial equalities.
+	local changes = false
+	local equalities = {} -- Stack of indices where equalities are found.
+	local equalitiesLength = 0 -- Keeping our own length var is faster.
+	local lastEquality: string? = nil
+	-- Always equal to diffs[equalities[equalitiesLength]].value
+	local pointer = 1 -- Index of current position.
+	-- Number of characters that changed prior to the equality.
+	local length_insertions1 = 0
+	local length_deletions1 = 0
+	-- Number of characters that changed after the equality.
+	local length_insertions2 = 0
+	local length_deletions2 = 0
+
+	while diffs[pointer] do
+		if diffs[pointer].actionType == StringDiff.ActionTypes.Equal then -- Equality found.
+			equalitiesLength = equalitiesLength + 1
+			equalities[equalitiesLength] = pointer
+			length_insertions1 = length_insertions2
+			length_deletions1 = length_deletions2
+			length_insertions2 = 0
+			length_deletions2 = 0
+			lastEquality = diffs[pointer].value
+		else -- An insertion or deletion.
+			if diffs[pointer].actionType == StringDiff.ActionTypes.Insert then
+				length_insertions2 = length_insertions2 + #diffs[pointer].value
+			else
+				length_deletions2 = length_deletions2 + #diffs[pointer].value
+			end
+			-- Eliminate an equality that is smaller or equal to the edits on both
+			-- sides of it.
+			if
+				lastEquality
+				and (#lastEquality <= math.max(length_insertions1, length_deletions1))
+				and (#lastEquality <= math.max(length_insertions2, length_deletions2))
+			then
+				-- Duplicate record.
+				table.insert(
+					diffs,
+					equalities[equalitiesLength],
+					{ actionType = StringDiff.ActionTypes.Delete, value = lastEquality }
+				)
+				-- Change second copy to insert.
+				diffs[equalities[equalitiesLength] + 1].actionType = StringDiff.ActionTypes.Insert
+				-- Throw away the equality we just deleted.
+				equalitiesLength = equalitiesLength - 1
+				-- Throw away the previous equality (it needs to be reevaluated).
+				equalitiesLength = equalitiesLength - 1
+				pointer = (equalitiesLength > 0) and equalities[equalitiesLength] or 0
+				length_insertions1, length_deletions1 = 0, 0 -- Reset the counters.
+				length_insertions2, length_deletions2 = 0, 0
+				lastEquality = nil
+				changes = true
+			end
+		end
+		pointer = pointer + 1
+	end
+
+	-- Normalize the diff.
+	if changes then
+		StringDiff._reorderAndMerge(diffs)
+	end
+	StringDiff._cleanupSemanticLossless(diffs)
+
+	-- Find any overlaps between deletions and insertions.
+	-- e.g: <del>abcxxx</del><ins>xxxdef</ins>
+	--   -> <del>abc</del>xxx<ins>def</ins>
+	-- e.g: <del>xxxabc</del><ins>defxxx</ins>
+	--   -> <ins>def</ins>xxx<del>abc</del>
+	-- Only extract an overlap if it is as big as the edit ahead or behind it.
+	pointer = 2
+	while diffs[pointer] do
+		if
+			diffs[pointer - 1].actionType == StringDiff.ActionTypes.Delete
+			and diffs[pointer].actionType == StringDiff.ActionTypes.Insert
+		then
+			local deletion = diffs[pointer - 1].value
+			local insertion = diffs[pointer].value
+			local overlap_length1 = StringDiff._commonOverlap(deletion, insertion)
+			local overlap_length2 = StringDiff._commonOverlap(insertion, deletion)
+			if overlap_length1 >= overlap_length2 then
+				if overlap_length1 >= #deletion / 2 or overlap_length1 >= #insertion / 2 then
+					-- Overlap found.  Insert an equality and trim the surrounding edits.
+					table.insert(
+						diffs,
+						pointer,
+						{ actionType = StringDiff.ActionTypes.Equal, value = string.sub(insertion, 1, overlap_length1) }
+					)
+					diffs[pointer - 1].value = string.sub(deletion, 1, #deletion - overlap_length1)
+					diffs[pointer + 1].value = string.sub(insertion, overlap_length1 + 1)
+					pointer = pointer + 1
+				end
+			else
+				if overlap_length2 >= #deletion / 2 or overlap_length2 >= #insertion / 2 then
+					-- Reverse overlap found.
+					-- Insert an equality and swap and trim the surrounding edits.
+					table.insert(
+						diffs,
+						pointer,
+						{ actionType = StringDiff.ActionTypes.Equal, value = string.sub(deletion, 1, overlap_length2) }
+					)
+					diffs[pointer - 1] = {
+						actionType = StringDiff.ActionTypes.Insert,
+						value = string.sub(insertion, 1, #insertion - overlap_length2),
+					}
+					diffs[pointer + 1] = {
+						actionType = StringDiff.ActionTypes.Delete,
+						value = string.sub(deletion, overlap_length2 + 1),
+					}
+					pointer = pointer + 1
+				end
+			end
+			pointer = pointer + 1
+		end
+		pointer = pointer + 1
+	end
+
 	return diffs
 end
 
@@ -124,51 +304,164 @@ function StringDiff._sharedSuffix(text1: string, text2: string): number
 	return pointerMid
 end
 
-function StringDiff._computeDiff(text1: string, text2: string): Diffs
-	-- Assumes that the prefix and suffix have already been trimmed off
-	-- and shortcut returns have been made so these texts must be different
+function StringDiff._commonOverlap(text1: string, text2: string): number
+	-- Determine if the suffix of one string is the prefix of another.
 
-	local text1Length, text2Length = #text1, #text2
-
-	if text1Length == 0 then
-		-- It's simply inserting all of text2 into text1
-		return { { actionType = StringDiff.ActionTypes.Insert, value = text2 } }
+	-- Cache the text lengths to prevent multiple calls.
+	local text1_length = #text1
+	local text2_length = #text2
+	-- Eliminate the null case.
+	if text1_length == 0 or text2_length == 0 then
+		return 0
+	end
+	-- Truncate the longer string.
+	if text1_length > text2_length then
+		text1 = string.sub(text1, text1_length - text2_length + 1)
+	elseif text1_length < text2_length then
+		text2 = string.sub(text2, 1, text1_length)
+	end
+	local text_length = math.min(text1_length, text2_length)
+	-- Quick check for the worst case.
+	if text1 == text2 then
+		return text_length
 	end
 
-	if text2Length == 0 then
-		-- It's simply deleting all of text1
-		return { { actionType = StringDiff.ActionTypes.Delete, value = text1 } }
-	end
-
-	local longText = if text1Length > text2Length then text1 else text2
-	local shortText = if text1Length > text2Length then text2 else text1
-	local shortTextLength = #shortText
-
-	-- Shortcut if the shorter string exists entirely inside the longer one
-	local indexOf = if shortTextLength == 0 then nil else string.find(longText, shortText, 1, true)
-	if indexOf ~= nil then
-		local diffs = {
-			{ actionType = StringDiff.ActionTypes.Insert, value = string.sub(longText, 1, indexOf - 1) },
-			{ actionType = StringDiff.ActionTypes.Equal, value = shortText },
-			{ actionType = StringDiff.ActionTypes.Insert, value = string.sub(longText, indexOf + shortTextLength) },
-		}
-		-- Swap insertions for deletions if diff is reversed
-		if text1Length > text2Length then
-			diffs[1].actionType, diffs[3].actionType = StringDiff.ActionTypes.Delete, StringDiff.ActionTypes.Delete
+	-- Start by looking for a single character match
+	-- and increase length until no match is found.
+	-- Performance analysis: https://neil.fraser.name/news/2010/11/04/
+	local best = 0
+	local length = 1
+	while true do
+		local pattern = string.sub(text1, text_length - length + 1)
+		local found = string.find(text2, pattern, 1, true)
+		if found == nil then
+			return best
 		end
-		return diffs
+		length = length + found - 1
+		if found == 1 or string.sub(text1, text_length - length + 1) == string.sub(text2, 1, length) then
+			best = length
+			length = length + 1
+		end
+	end
+end
+
+function StringDiff._cleanupSemanticScore(one: string, two: string): number
+	-- Given two strings, compute a score representing whether the internal
+	-- boundary falls on logical boundaries.
+	-- Scores range from 6 (best) to 0 (worst).
+
+	if (#one == 0) or (#two == 0) then
+		-- Edges are the best.
+		return 6
 	end
 
-	if shortTextLength == 1 then
-		-- Single character string
-		-- After the previous shortcut, the character can't be an equality
-		return {
-			{ actionType = StringDiff.ActionTypes.Delete, value = text1 },
-			{ actionType = StringDiff.ActionTypes.Insert, value = text2 },
-		}
-	end
+	-- Each port of this function behaves slightly differently due to
+	-- subtle differences in each language's definition of things like
+	-- 'whitespace'.  Since this function's purpose is largely cosmetic,
+	-- the choice has been made to use each language's native features
+	-- rather than force total conformity.
+	local char1 = string.sub(one, -1)
+	local char2 = string.sub(two, 1, 1)
+	local nonAlphaNumeric1 = string.match(char1, "%W")
+	local nonAlphaNumeric2 = string.match(char2, "%W")
+	local whitespace1 = nonAlphaNumeric1 and string.match(char1, "%s")
+	local whitespace2 = nonAlphaNumeric2 and string.match(char2, "%s")
+	local lineBreak1 = whitespace1 and string.match(char1, "%c")
+	local lineBreak2 = whitespace2 and string.match(char2, "%c")
+	local blankLine1 = lineBreak1 and string.match(one, "\n\r?\n$")
+	local blankLine2 = lineBreak2 and string.match(two, "^\r?\n\r?\n")
 
-	return StringDiff._bisect(text1, text2)
+	if blankLine1 or blankLine2 then
+		-- Five points for blank lines.
+		return 5
+	elseif lineBreak1 or lineBreak2 then
+		-- Four points for line breaks
+		-- DEVIATION: Prefer to start on a line break instead of end on it
+		return if lineBreak1 then 4 else 4.5
+	elseif nonAlphaNumeric1 and not whitespace1 and whitespace2 then
+		-- Three points for end of sentences.
+		return 3
+	elseif whitespace1 or whitespace2 then
+		-- Two points for whitespace.
+		return 2
+	elseif nonAlphaNumeric1 or nonAlphaNumeric2 then
+		-- One point for non-alphanumeric.
+		return 1
+	end
+	return 0
+end
+
+function StringDiff._cleanupSemanticLossless(diffs: Diffs)
+	-- Look for single edits surrounded on both sides by equalities
+	-- which can be shifted sideways to align the edit to a word boundary.
+	-- e.g: The c<ins>at c</ins>ame. -> The <ins>cat </ins>came.
+
+	local pointer = 2
+	-- Intentionally ignore the first and last element (don't need checking).
+	while diffs[pointer + 1] do
+		local prevDiff, nextDiff = diffs[pointer - 1], diffs[pointer + 1]
+		if
+			(prevDiff.actionType == StringDiff.ActionTypes.Equal)
+			and (nextDiff.actionType == StringDiff.ActionTypes.Equal)
+		then
+			-- This is a single edit surrounded by equalities.
+			local diff = diffs[pointer]
+
+			local equality1 = prevDiff.value
+			local edit = diff.value
+			local equality2 = nextDiff.value
+
+			-- First, shift the edit as far left as possible.
+			local commonOffset = StringDiff._sharedSuffix(equality1, edit)
+			if commonOffset > 0 then
+				local commonString = string.sub(edit, -commonOffset)
+				equality1 = string.sub(equality1, 1, -commonOffset - 1)
+				edit = commonString .. string.sub(edit, 1, -commonOffset - 1)
+				equality2 = commonString .. equality2
+			end
+
+			-- Second, step character by character right, looking for the best fit.
+			local bestEquality1 = equality1
+			local bestEdit = edit
+			local bestEquality2 = equality2
+			local bestScore = StringDiff._cleanupSemanticScore(equality1, edit)
+				+ StringDiff._cleanupSemanticScore(edit, equality2)
+
+			while string.byte(edit, 1) == string.byte(equality2, 1) do
+				equality1 = equality1 .. string.sub(edit, 1, 1)
+				edit = string.sub(edit, 2) .. string.sub(equality2, 1, 1)
+				equality2 = string.sub(equality2, 2)
+				local score = StringDiff._cleanupSemanticScore(equality1, edit)
+					+ StringDiff._cleanupSemanticScore(edit, equality2)
+				-- The > (rather than >=) encourages leading rather than trailing whitespace on edits.
+				-- I just think it looks better for indentation changes to start the line,
+				-- since then indenting several lines all have aligned diffs at the start
+				if score > bestScore then
+					bestScore = score
+					bestEquality1 = equality1
+					bestEdit = edit
+					bestEquality2 = equality2
+				end
+			end
+			if prevDiff.value ~= bestEquality1 then
+				-- We have an improvement, save it back to the diff.
+				if #bestEquality1 > 0 then
+					diffs[pointer - 1].value = bestEquality1
+				else
+					table.remove(diffs, pointer - 1)
+					pointer = pointer - 1
+				end
+				diffs[pointer].value = bestEdit
+				if #bestEquality2 > 0 then
+					diffs[pointer + 1].value = bestEquality2
+				else
+					table.remove(diffs, pointer + 1)
+					pointer = pointer - 1
+				end
+			end
+		end
+		pointer = pointer + 1
+	end
 end
 
 function StringDiff._bisect(text1: string, text2: string): Diffs

plugin/src/App/Components/StringDiffVisualizer/init.lua

@@ -5,15 +5,15 @@ local Packages = Rojo.Packages
 local Roact = require(Packages.Roact)
 local Log = require(Packages.Log)
 local Highlighter = require(Packages.Highlighter)
+Highlighter.matchStudioSettings()
 local StringDiff = require(script:FindFirstChild("StringDiff"))
 
 local Timer = require(Plugin.Timer)
 local Theme = require(Plugin.App.Theme)
 local getTextBoundsAsync = require(Plugin.App.getTextBoundsAsync)
 
-local CodeLabel = require(Plugin.App.Components.CodeLabel)
 local BorderedContainer = require(Plugin.App.Components.BorderedContainer)
-local ScrollingFrame = require(Plugin.App.Components.ScrollingFrame)
+local VirtualScroller = require(Plugin.App.Components.VirtualScroller)
 
 local e = Roact.createElement
 
@@ -21,26 +21,29 @@ local StringDiffVisualizer = Roact.Component:extend("StringDiffVisualizer")
 
 function StringDiffVisualizer:init()
 	self.scriptBackground, self.setScriptBackground = Roact.createBinding(Color3.fromRGB(0, 0, 0))
-	self.contentSize, self.setContentSize = Roact.createBinding(Vector2.new(0, 0))
+	self.updateEvent = Instance.new("BindableEvent")
+	self.lineHeight, self.setLineHeight = Roact.createBinding(15)
+	self.canvasPosition, self.setCanvasPosition = Roact.createBinding(Vector2.zero)
+	self.windowWidth, self.setWindowWidth = Roact.createBinding(math.huge)
 
 	-- Ensure that the script background is up to date with the current theme
 	self.themeChangedConnection = settings().Studio.ThemeChanged:Connect(function()
-		task.defer(function()
-			-- Defer to allow Highlighter to process the theme change first
+		-- Delay to allow Highlighter to process the theme change first
+		task.delay(1 / 20, function()
 			self:updateScriptBackground()
+			self:updateDiffs()
+			-- Rerender the virtual list elements
+			self.updateEvent:Fire()
 		end)
 	end)
 
 	self:updateScriptBackground()
-
-	self:setState({
-		add = {},
-		remove = {},
-	})
+	self:updateDiffs()
 end
 
 function StringDiffVisualizer:willUnmount()
 	self.themeChangedConnection:Disconnect()
+	self.updateEvent:Destroy()
 end
 
 function StringDiffVisualizer:updateScriptBackground()
@@ -51,96 +54,188 @@ function StringDiffVisualizer:updateScriptBackground()
 end
 
 function StringDiffVisualizer:didUpdate(previousProps)
-	if previousProps.oldString ~= self.props.oldString or previousProps.newString ~= self.props.newString then
-		local add, remove = self:calculateDiffLines()
-		self:setState({
-			add = add,
-			remove = remove,
-		})
+	if
+		previousProps.currentString ~= self.props.currentString
+		or previousProps.incomingString ~= self.props.incomingString
+	then
+		self:updateDiffs()
 	end
 end
 
-function StringDiffVisualizer:calculateContentSize(theme)
-	local oldString, newString = self.props.oldString, self.props.newString
-
-	local oldStringBounds = getTextBoundsAsync(oldString, theme.Font.Code, theme.TextSize.Code, math.huge)
-	local newStringBounds = getTextBoundsAsync(newString, theme.Font.Code, theme.TextSize.Code, math.huge)
-
-	self.setContentSize(
-		Vector2.new(math.max(oldStringBounds.X, newStringBounds.X), math.max(oldStringBounds.Y, newStringBounds.Y))
-	)
-end
-
-function StringDiffVisualizer:calculateDiffLines()
-	Timer.start("StringDiffVisualizer:calculateDiffLines")
-	local oldString, newString = self.props.oldString, self.props.newString
+function StringDiffVisualizer:updateDiffs()
+	Timer.start("StringDiffVisualizer:updateDiffs")
+	local currentString, incomingString = self.props.currentString, self.props.incomingString
 
 	-- Diff the two texts
 	local startClock = os.clock()
-	local diffs = StringDiff.findDiffs(oldString, newString)
+	local diffs =
+		StringDiff.findDiffs((string.gsub(currentString, "\t", "    ")), (string.gsub(incomingString, "\t", "    ")))
 	local stopClock = os.clock()
 
 	Log.trace(
 		"Diffing {} byte and {} byte strings took {} microseconds and found {} diff sections",
-		#oldString,
-		#newString,
+		#currentString,
+		#incomingString,
 		math.round((stopClock - startClock) * 1000 * 1000),
 		#diffs
 	)
 
-	-- Determine which lines to highlight
-	local add, remove = {}, {}
+	-- Build the rich text lines
+	local currentRichTextLines = Highlighter.buildRichTextLines({
+		src = currentString,
+	})
+	local incomingRichTextLines = Highlighter.buildRichTextLines({
+		src = incomingString,
+	})
 
-	local oldLineNum, newLineNum = 1, 1
+	local maxLines = math.max(#currentRichTextLines, #incomingRichTextLines)
+
+	-- Find the diff locations
+	local currentDiffs, incomingDiffs = {}, {}
+	local firstDiffLineNum = 0
+
+	local currentLineNum, incomingLineNum = 1, 1
+	local currentIdx, incomingIdx = 1, 1
 	for _, diff in diffs do
 		local actionType, text = diff.actionType, diff.value
-		local lines = select(2, string.gsub(text, "\n", "\n"))
+		local lineCount = select(2, string.gsub(text, "\n", "\n"))
+		local lines = string.split(text, "\n")
 
 		if actionType == StringDiff.ActionTypes.Equal then
-			oldLineNum += lines
-			newLineNum += lines
-		elseif actionType == StringDiff.ActionTypes.Insert then
-			if lines > 0 then
-				local textLines = string.split(text, "\n")
-				for i, textLine in textLines do
-					if string.match(textLine, "%S") then
-						add[newLineNum + i - 1] = true
-					end
-				end
+			if lineCount > 0 then
+				-- Jump cursor ahead to last line
+				currentLineNum += lineCount
+				incomingLineNum += lineCount
+				currentIdx = #lines[#lines]
+				incomingIdx = #lines[#lines]
 			else
-				if string.match(text, "%S") then
-					add[newLineNum] = true
-				end
+				-- Move along this line
+				currentIdx += #text
+				incomingIdx += #text
+			end
+
+			continue
+		end
+
+		if actionType == StringDiff.ActionTypes.Insert then
+			if firstDiffLineNum == 0 then
+				firstDiffLineNum = incomingLineNum
+			end
+
+			for i, lineText in lines do
+				if i > 1 then
+					-- Move to next line
+					incomingLineNum += 1
+					incomingIdx = 0
+				end
+				if not incomingDiffs[incomingLineNum] then
+					incomingDiffs[incomingLineNum] = {}
+				end
+				-- Mark these characters on this line
+				table.insert(incomingDiffs[incomingLineNum], {
+					start = incomingIdx,
+					stop = incomingIdx + #lineText,
+				})
+				incomingIdx += #lineText
 			end
-			newLineNum += lines
 		elseif actionType == StringDiff.ActionTypes.Delete then
-			if lines > 0 then
-				local textLines = string.split(text, "\n")
-				for i, textLine in textLines do
-					if string.match(textLine, "%S") then
-						remove[oldLineNum + i - 1] = true
-					end
-				end
-			else
-				if string.match(text, "%S") then
-					remove[oldLineNum] = true
-				end
+			if firstDiffLineNum == 0 then
+				firstDiffLineNum = currentLineNum
+			end
+
+			for i, lineText in lines do
+				if i > 1 then
+					-- Move to next line
+					currentLineNum += 1
+					currentIdx = 0
+				end
+				if not currentDiffs[currentLineNum] then
+					currentDiffs[currentLineNum] = {}
+				end
+				-- Mark these characters on this line
+				table.insert(currentDiffs[currentLineNum], {
+					start = currentIdx,
+					stop = currentIdx + #lineText,
+				})
+				currentIdx += #lineText
 			end
-			oldLineNum += lines
 		else
 			Log.warn("Unknown diff action: {} {}", actionType, text)
 		end
 	end
 
 	Timer.stop()
-	return add, remove
+
+	self:setState({
+		maxLines = maxLines,
+		currentRichTextLines = currentRichTextLines,
+		incomingRichTextLines = incomingRichTextLines,
+		currentDiffs = currentDiffs,
+		incomingDiffs = incomingDiffs,
+	})
+
+	-- Scroll to the first diff line
+	task.defer(self.setCanvasPosition, Vector2.new(0, math.max(0, (firstDiffLineNum - 4) * 16)))
 end
 
 function StringDiffVisualizer:render()
-	local oldString, newString = self.props.oldString, self.props.newString
+	local currentDiffs, incomingDiffs = self.state.currentDiffs, self.state.incomingDiffs
+	local currentRichTextLines, incomingRichTextLines =
+		self.state.currentRichTextLines, self.state.incomingRichTextLines
+	local maxLines = self.state.maxLines
 
 	return Theme.with(function(theme)
-		self:calculateContentSize(theme)
+		self.setLineHeight(theme.TextSize.Code)
+
+		-- Calculate the width of the canvas
+		-- (One line at a time to avoid the char limit of getTextBoundsAsync)
+		local canvasWidth = 0
+		for i = 1, maxLines do
+			local currentLine = currentRichTextLines[i]
+			if currentLine and string.find(currentLine, "%S") then
+				local bounds = getTextBoundsAsync(currentLine, theme.Font.Code, theme.TextSize.Code, math.huge, true)
+				if bounds.X > canvasWidth then
+					canvasWidth = bounds.X
+				end
+			end
+			local incomingLine = incomingRichTextLines[i]
+			if incomingLine and string.find(incomingLine, "%S") then
+				local bounds = getTextBoundsAsync(incomingLine, theme.Font.Code, theme.TextSize.Code, math.huge, true)
+				if bounds.X > canvasWidth then
+					canvasWidth = bounds.X
+				end
+			end
+		end
+
+		local lineNumberWidth =
+			getTextBoundsAsync(tostring(maxLines), theme.Font.Code, theme.TextSize.Body, math.huge, true).X
+
+		canvasWidth += lineNumberWidth + 12
+
+		local removalScrollMarkers = {}
+		local insertionScrollMarkers = {}
+		for lineNum in currentDiffs do
+			table.insert(
+				removalScrollMarkers,
+				e("Frame", {
+					Size = UDim2.fromScale(0.5, 1 / maxLines),
+					Position = UDim2.fromScale(0, (lineNum - 1) / maxLines),
+					BorderSizePixel = 0,
+					BackgroundColor3 = theme.Diff.Background.Remove,
+				})
+			)
+		end
+		for lineNum in incomingDiffs do
+			table.insert(
+				insertionScrollMarkers,
+				e("Frame", {
+					Size = UDim2.fromScale(0.5, 1 / maxLines),
+					Position = UDim2.fromScale(0.5, (lineNum - 1) / maxLines),
+					BorderSizePixel = 0,
+					BackgroundColor3 = theme.Diff.Background.Add,
+				})
+			)
+		end
 
 		return e(BorderedContainer, {
 			size = self.props.size,
@@ -159,43 +254,196 @@ function StringDiffVisualizer:render()
 					CornerRadius = UDim.new(0, 5),
 				}),
 			}),
-			Separator = e("Frame", {
-				Size = UDim2.new(0, 2, 1, 0),
-				Position = UDim2.new(0.5, 0, 0, 0),
-				AnchorPoint = Vector2.new(0.5, 0),
-				BorderSizePixel = 0,
-				BackgroundColor3 = theme.BorderedContainer.BorderColor,
-				BackgroundTransparency = 0.5,
-			}),
-			Old = e(ScrollingFrame, {
-				position = UDim2.new(0, 2, 0, 2),
-				size = UDim2.new(0.5, -7, 1, -4),
-				scrollingDirection = Enum.ScrollingDirection.XY,
-				transparency = self.props.transparency,
-				contentSize = self.contentSize,
+			Main = e("Frame", {
+				Size = UDim2.new(1, -10, 1, -2),
+				Position = UDim2.new(0, 2, 0, 2),
+				BackgroundTransparency = 1,
+				[Roact.Change.AbsoluteSize] = function(rbx)
+					self.setWindowWidth(rbx.AbsoluteSize.X * 0.5 - 10)
+				end,
 			}, {
-				Source = e(CodeLabel, {
-					size = UDim2.new(1, 0, 1, 0),
+				Separator = e("Frame", {
+					Size = UDim2.new(0, 2, 1, 0),
+					Position = UDim2.new(0.5, 0, 0, 0),
+					AnchorPoint = Vector2.new(0.5, 0),
+					BorderSizePixel = 0,
+					BackgroundColor3 = theme.BorderedContainer.BorderColor,
+					BackgroundTransparency = 0.5,
+				}),
+				Current = e(VirtualScroller, {
 					position = UDim2.new(0, 0, 0, 0),
-					text = oldString,
-					lineBackground = theme.Diff.Background.Remove,
-					markedLines = self.state.remove,
+					size = UDim2.new(0.5, -1, 1, 0),
+					transparency = self.props.transparency,
+					count = maxLines,
+					updateEvent = self.updateEvent.Event,
+					canvasWidth = canvasWidth,
+					canvasPosition = self.canvasPosition,
+					onCanvasPositionChanged = self.setCanvasPosition,
+					render = function(i)
+						local lineDiffs = currentDiffs[i]
+						local diffFrames = table.create(if lineDiffs then #lineDiffs else 0)
+
+						-- Show diff markers over the specific changed characters
+						if lineDiffs then
+							local charWidth = math.round(theme.TextSize.Code * 0.5)
+							for diffIdx, diff in lineDiffs do
+								local start, stop = diff.start, diff.stop
+								diffFrames[diffIdx] = e("Frame", {
+									Size = if #lineDiffs == 1
+											and start == 0
+											and stop == 0
+										then UDim2.fromScale(1, 1)
+										else UDim2.new(
+											0,
+											math.max(charWidth * (stop - start), charWidth * 0.4),
+											1,
+											0
+										),
+									Position = UDim2.fromOffset(charWidth * start, 0),
+									BackgroundColor3 = theme.Diff.Background.Remove,
+									BackgroundTransparency = 0.85,
+									BorderSizePixel = 0,
+									ZIndex = -1,
+								})
+							end
+						end
+
+						return Roact.createFragment({
+							LineNumber = e("TextLabel", {
+								Size = UDim2.new(0, lineNumberWidth + 8, 1, 0),
+								Text = i,
+								BackgroundColor3 = Color3.new(0, 0, 0),
+								BackgroundTransparency = 0.9,
+								BorderSizePixel = 0,
+								FontFace = theme.Font.Code,
+								TextSize = theme.TextSize.Body,
+								TextColor3 = if lineDiffs then theme.Diff.Background.Remove else theme.SubTextColor,
+								TextXAlignment = Enum.TextXAlignment.Right,
+							}, {
+								Padding = e("UIPadding", { PaddingRight = UDim.new(0, 6) }),
+							}),
+							Content = e("Frame", {
+								Size = UDim2.new(1, -(lineNumberWidth + 10), 1, 0),
+								Position = UDim2.fromScale(1, 0),
+								AnchorPoint = Vector2.new(1, 0),
+								BackgroundColor3 = theme.Diff.Background.Remove,
+								BackgroundTransparency = if lineDiffs then 0.95 else 1,
+								BorderSizePixel = 0,
+							}, {
+								CodeLabel = e("TextLabel", {
+									Size = UDim2.fromScale(1, 1),
+									Position = UDim2.fromScale(0, 0),
+									Text = currentRichTextLines[i] or "",
+									RichText = true,
+									BackgroundTransparency = 1,
+									BorderSizePixel = 0,
+									FontFace = theme.Font.Code,
+									TextSize = theme.TextSize.Code,
+									TextXAlignment = Enum.TextXAlignment.Left,
+									TextYAlignment = Enum.TextYAlignment.Top,
+									TextColor3 = Color3.fromRGB(255, 255, 255),
+								}),
+								DiffFrames = Roact.createFragment(diffFrames),
+							}),
+						})
+					end,
+					getHeightBinding = function()
+						return self.lineHeight
+					end,
+				}),
+				Incoming = e(VirtualScroller, {
+					position = UDim2.new(0.5, 1, 0, 0),
+					size = UDim2.new(0.5, -1, 1, 0),
+					transparency = self.props.transparency,
+					count = maxLines,
+					updateEvent = self.updateEvent.Event,
+					canvasWidth = canvasWidth,
+					canvasPosition = self.canvasPosition,
+					onCanvasPositionChanged = self.setCanvasPosition,
+					render = function(i)
+						local lineDiffs = incomingDiffs[i]
+						local diffFrames = table.create(if lineDiffs then #lineDiffs else 0)
+
+						-- Show diff markers over the specific changed characters
+						if lineDiffs then
+							local charWidth = math.round(theme.TextSize.Code * 0.5)
+							for diffIdx, diff in lineDiffs do
+								local start, stop = diff.start, diff.stop
+								diffFrames[diffIdx] = e("Frame", {
+									Size = if #lineDiffs == 1
+											and start == 0
+											and stop == 0
+										then UDim2.fromScale(1, 1)
+										else UDim2.new(
+											0,
+											math.max(charWidth * (stop - start), charWidth * 0.4),
+											1,
+											0
+										),
+									Position = UDim2.fromOffset(charWidth * start, 0),
+									BackgroundColor3 = theme.Diff.Background.Add,
+									BackgroundTransparency = 0.85,
+									BorderSizePixel = 0,
+									ZIndex = -1,
+								})
+							end
+						end
+
+						return Roact.createFragment({
+							LineNumber = e("TextLabel", {
+								Size = UDim2.new(0, lineNumberWidth + 8, 1, 0),
+								Text = i,
+								BackgroundColor3 = Color3.new(0, 0, 0),
+								BackgroundTransparency = 0.9,
+								BorderSizePixel = 0,
+								FontFace = theme.Font.Code,
+								TextSize = theme.TextSize.Body,
+								TextColor3 = if lineDiffs then theme.Diff.Background.Add else theme.SubTextColor,
+								TextXAlignment = Enum.TextXAlignment.Right,
+							}, {
+								Padding = e("UIPadding", { PaddingRight = UDim.new(0, 6) }),
+							}),
+							Content = e("Frame", {
+								Size = UDim2.new(1, -(lineNumberWidth + 10), 1, 0),
+								Position = UDim2.fromScale(1, 0),
+								AnchorPoint = Vector2.new(1, 0),
+								BackgroundColor3 = theme.Diff.Background.Add,
+								BackgroundTransparency = if lineDiffs then 0.95 else 1,
+								BorderSizePixel = 0,
+							}, {
+								CodeLabel = e("TextLabel", {
+									Size = UDim2.fromScale(1, 1),
+									Position = UDim2.fromScale(0, 0),
+									Text = incomingRichTextLines[i] or "",
+									RichText = true,
+									BackgroundColor3 = theme.Diff.Background.Add,
+									BackgroundTransparency = 1,
+									BorderSizePixel = 0,
+									FontFace = theme.Font.Code,
+									TextSize = theme.TextSize.Code,
+									TextXAlignment = Enum.TextXAlignment.Left,
+									TextYAlignment = Enum.TextYAlignment.Top,
+									TextColor3 = Color3.fromRGB(255, 255, 255),
+								}),
+								DiffFrames = Roact.createFragment(diffFrames),
+							}),
+						})
+					end,
+					getHeightBinding = function()
+						return self.lineHeight
+					end,
 				}),
 			}),
-			New = e(ScrollingFrame, {
-				position = UDim2.new(0.5, 5, 0, 2),
-				size = UDim2.new(0.5, -7, 1, -4),
-				scrollingDirection = Enum.ScrollingDirection.XY,
-				transparency = self.props.transparency,
-				contentSize = self.contentSize,
+			ScrollMarkers = e("Frame", {
+				Size = self.windowWidth:map(function(windowWidth)
+					return UDim2.new(0, 8, 1, -4 - (if canvasWidth > windowWidth then 10 else 0))
+				end),
+				Position = UDim2.new(1, -2, 0, 2),
+				AnchorPoint = Vector2.new(1, 0),
+				BackgroundTransparency = 1,
 			}, {
-				Source = e(CodeLabel, {
-					size = UDim2.new(1, 0, 1, 0),
-					position = UDim2.new(0, 0, 0, 0),
-					text = newString,
-					lineBackground = theme.Diff.Background.Add,
-					markedLines = self.state.add,
-				}),
+				insertions = Roact.createFragment(insertionScrollMarkers),
+				removals = Roact.createFragment(removalScrollMarkers),
 			}),
 		})
 	end)

plugin/src/App/Components/VirtualScroller.lua

@@ -15,8 +15,10 @@ local VirtualScroller = Roact.Component:extend("VirtualScroller")
 function VirtualScroller:init()
 	self.scrollFrameRef = Roact.createRef()
 	self:setState({
-		WindowSize = Vector2.new(),
-		CanvasPosition = Vector2.new(),
+		WindowSize = Vector2.zero,
+		CanvasPosition = if self.props.canvasPosition
+			then self.props.canvasPosition:getValue() or Vector2.zero
+			else Vector2.zero,
 	})
 
 	self.totalCanvas, self.setTotalCanvas = Roact.createBinding(0)
@@ -41,6 +43,10 @@ function VirtualScroller:didMount()
 
 	local canvasPositionSignal = rbx:GetPropertyChangedSignal("CanvasPosition")
 	self.canvasPositionChanged = canvasPositionSignal:Connect(function()
+		if self.props.onCanvasPositionChanged then
+			pcall(self.props.onCanvasPositionChanged, rbx.CanvasPosition)
+		end
+
 		if math.abs(rbx.CanvasPosition.Y - self.state.CanvasPosition.Y) > 5 then
 			self:setState({ CanvasPosition = rbx.CanvasPosition })
 			self:refresh()
@@ -134,8 +140,9 @@ function VirtualScroller:render()
 			BackgroundColor3 = props.backgroundColor3 or theme.BorderedContainer.BackgroundColor,
 			BorderColor3 = props.borderColor3 or theme.BorderedContainer.BorderColor,
 			CanvasSize = self.totalCanvas:map(function(s)
-				return UDim2.fromOffset(0, s)
+				return UDim2.fromOffset(props.canvasWidth or 0, s)
 			end),
+			CanvasPosition = self.props.canvasPosition,
 			ScrollBarThickness = 9,
 			ScrollBarImageColor3 = theme.ScrollBarColor,
 			ScrollBarImageTransparency = props.transparency:map(function(value)
@@ -146,7 +153,7 @@ function VirtualScroller:render()
 			BottomImage = Assets.Images.ScrollBar.Bottom,
 
 			ElasticBehavior = Enum.ElasticBehavior.Always,
-			ScrollingDirection = Enum.ScrollingDirection.Y,
+			ScrollingDirection = Enum.ScrollingDirection.XY,
 			VerticalScrollBarInset = Enum.ScrollBarInset.ScrollBar,
 			[Roact.Ref] = self.scrollFrameRef,
 		}, {

plugin/src/App/StatusPages/Confirming.lua

@@ -23,8 +23,8 @@ function ConfirmingPage:init()
 
 	self:setState({
 		showingStringDiff = false,
-		oldString = "",
-		newString = "",
+		currentString = "",
+		incomingString = "",
 		showingTableDiff = false,
 		oldTable = {},
 		newTable = {},
@@ -56,11 +56,11 @@ function ConfirmingPage:render()
 
 				patchTree = self.props.patchTree,
 
-				showStringDiff = function(oldString: string, newString: string)
+				showStringDiff = function(currentString: string, incomingString: string)
 					self:setState({
 						showingStringDiff = true,
-						oldString = oldString,
-						newString = newString,
+						currentString = currentString,
+						incomingString = incomingString,
 					})
 				end,
 				showTableDiff = function(oldTable: { [any]: any? }, newTable: { [any]: any? })
@@ -167,8 +167,8 @@ function ConfirmingPage:render()
 							anchorPoint = Vector2.new(0, 0),
 							transparency = self.props.transparency,
 
-							oldString = self.state.oldString,
-							newString = self.state.newString,
+							currentString = self.state.currentString,
+							incomingString = self.state.incomingString,
 						}),
 					}),
 				}),

plugin/src/App/StatusPages/Connected.lua

@@ -307,8 +307,8 @@ function ConnectedPage:init()
 		renderChanges = false,
 		hoveringChangeInfo = false,
 		showingStringDiff = false,
-		oldString = "",
-		newString = "",
+		currentString = "",
+		incomingString = "",
 	})
 
 	self.changeInfoText, self.setChangeInfoText = Roact.createBinding("")
@@ -511,11 +511,11 @@ function ConnectedPage:render()
 							patchData = self.props.patchData,
 							patchTree = self.props.patchTree,
 							serveSession = self.props.serveSession,
-							showStringDiff = function(oldString: string, newString: string)
+							showStringDiff = function(currentString: string, incomingString: string)
 								self:setState({
 									showingStringDiff = true,
-									oldString = oldString,
-									newString = newString,
+									currentString = currentString,
+									incomingString = incomingString,
 								})
 							end,
 							showTableDiff = function(oldTable: { [any]: any? }, newTable: { [any]: any? })
@@ -566,8 +566,8 @@ function ConnectedPage:render()
 							anchorPoint = Vector2.new(0, 0),
 							transparency = self.props.transparency,
 
-							oldString = self.state.oldString,
-							newString = self.state.newString,
+							currentString = self.state.currentString,
+							incomingString = self.state.incomingString,
 						}),
 					}),
 				}),