Diff view crash/failure

[OldStarchy]

Does this issue occur when all extensions are disabled?: Yes

• VS Code Version: 1.116.0 (user setup)
• OS Version: Windows_NT x64 10.0.26100

There is a bug in this section of code (approximately) that origLine is sometimes 0, causing areLinesSimilar to be called with an undefined.

vscode/src/vs/editor/common/diff/defaultLinesDiffComputer/computeMovedLines.ts

Lines 207 to 219 in 7049f52

	for (extendToTop = 0; extendToTop < linesAbove; extendToTop++) {
	const origLine = move.original.startLineNumber - extendToTop - 1;
	const modLine = move.modified.startLineNumber - extendToTop - 1;
	if (origLine > originalLines.length || modLine > modifiedLines.length) {
	break;
	}
	if (modifiedSet.contains(modLine) || originalSet.contains(origLine)) {
	break;
	}
	if (!areLinesSimilar(originalLines[origLine - 1], modifiedLines[modLine - 1], timeout)) {
	break;
	}
	}

I'm not sure of the exact conditions, but the code samples below trigger it.

Steps to Reproduce:

1. Stage a file with this content

   <UserControl x:Class="abc.def.ghi"
               xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
               xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
               xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
               xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
               mc:Ignorable="d"
               Background="{DynamicResource SomeColor}"
               d:DataContext="{d:DesignInstance abcd:Efgh, d:IsDesignTimeCreatable=False}"
               d:DesignHeight="600" d:DesignWidth="800">
   </UserControl>

2. Modify the file to this content

    <UserControl
        x:Class="abc.def.ghi"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        mc:Ignorable="d"
        Background="{DynamicResource SomeColor}"
        d:DataContext="{d:DesignInstance abcd:Efgh, d:IsDesignTimeCreatable=False}"
        d:DesignHeight="600"
        d:DesignWidth="800"
    >
    </UserControl>

3. Open the diff for unstaged changes (open the file from the "Changes" list in the source control view)
4. diff does not load

The "Window" output window shows

2026-04-16 11:26:12.390 [error] [Window] Cannot read properties of undefined (reading 'trim'): TypeError: Cannot read properties of undefined (reading 'trim')
    at os (vscode-file://vscode-app/c:/Users/OldStarchy/AppData/Local/Programs/Microsoft%20VS%20Code/41dd792b5e/resources/app/out/vs/editor/common/services/editorWebWorkerMain.js#editorWorkerService:424:3486)
    at hl (vscode-file://vscode-app/c:/Users/OldStarchy/AppData/Local/Programs/Microsoft%20VS%20Code/41dd792b5e/resources/app/out/vs/editor/common/services/editorWebWorkerMain.js#editorWorkerService:424:2727)
    at ss (vscode-file://vscode-app/c:/Users/OldStarchy/AppData/Local/Programs/Microsoft%20VS%20Code/41dd792b5e/resources/app/out/vs/editor/common/services/editorWebWorkerMain.js#editorWorkerService:423:6025)
    at $n.computeMoves (vscode-file://vscode-app/c:/Users/OldStarchy/AppData/Local/Programs/Microsoft%20VS%20Code/41dd792b5e/resources/app/out/vs/editor/common/services/editorWebWorkerMain.js#editorWorkerService:425:2267)
    at $n.computeDiff (vscode-file://vscode-app/c:/Users/OldStarchy/AppData/Local/Programs/Microsoft%20VS%20Code/41dd792b5e/resources/app/out/vs/editor/common/services/editorWebWorkerMain.js#editorWorkerService:425:1602)
    at r.computeDiff (vscode-file://vscode-app/c:/Users/OldStarchy/AppData/Local/Programs/Microsoft%20VS%20Code/41dd792b5e/resources/app/out/vs/editor/common/services/editorWebWorkerMain.js#editorWorkerService:429:3664)
    at r.$computeDiff (vscode-file://vscode-app/c:/Users/OldStarchy/AppData/Local/Programs/Microsoft%20VS%20Code/41dd792b5e/resources/app/out/vs/editor/common/services/editorWebWorkerMain.js#editorWorkerService:429:3515)
    at _n._handleMessage (vscode-file://vscode-app/c:/Users/OldStarchy/AppData/Local/Programs/Microsoft%20VS%20Code/41dd792b5e/resources/app/out/vs/editor/common/services/editorWebWorkerMain.js#editorWorkerService:402:65032)
    at Object.handleMessage (vscode-file://vscode-app/c:/Users/OldStarchy/AppData/Local/Programs/Microsoft%20VS%20Code/41dd792b5e/resources/app/out/vs/editor/common/services/editorWebWorkerMain.js#editorWorkerService:402:64530)
    at Mr._handleRequestMessage (vscode-file://vscode-app/c:/Users/OldStarchy/AppData/Local/Programs/Microsoft%20VS%20Code/41dd792b5e/resources/app/out/vs/editor/common/services/editorWebWorkerMain.js#editorWorkerService:402:63316)

Which I have traced to the above mentioned line of code and scenario.

Setting a breakpoint on line 210 above yields this

[vs-code-engineering]

Thanks for creating this issue! It looks like you may be using an old version of VS Code; the latest stable release is 1.116.0. Please try upgrading to the latest version and checking whether this issue remains.

Happy Coding!

[OldStarchy]

-			if (!areLinesSimilar(originalLines[origLine - 1], modifiedLines[modLine - 1], timeout)) {
+			if (!areLinesSimilar(originalLines[origLine - 1] ?? '', modifiedLines[modLine - 1] ?? '', timeout)) {

On lines 216 (and 236) makes it work, though it probably doesn't actually fix the cause. However '' does seem like a valid substitute for line[-1].