fix(diff): prevent stale fetch responses from showing wrong diff content

Add a generation counter to DiffViewerDialog so that when the user
navigates between files rapidly, only the most recent IPC response
updates the signals. Also guard MonacoDiffEditor setValue calls to
skip redundant updates when the model already holds the correct value.

Author: johannesjo

src/components/DiffViewerDialog.tsx

@@ -80,13 +80,16 @@ export function DiffViewerDialog(props: DiffViewerDialogProps) {
   const [hasChanges, setHasChanges] = createSignal(true);
   const [metadataOnly, setMetadataOnly] = createSignal(false);
 
+  let fetchGeneration = 0;
+
   createEffect(() => {
     const file = props.file;
     if (!file) return;
 
     const worktreePath = props.worktreePath;
     const projectRoot = props.projectRoot;
     const branchName = props.branchName;
+    const thisGen = ++fetchGeneration;
 
     setLoading(true);
     setError('');
@@ -112,6 +115,7 @@ export function DiffViewerDialog(props: DiffViewerDialogProps) {
         return { diff: '', oldContent: '', newContent: '' };
       })
       .then((result) => {
+        if (thisGen !== fetchGeneration) return;
         if (isBinaryDiff(result.diff)) {
           setBinary(true);
         } else {
@@ -122,8 +126,13 @@ export function DiffViewerDialog(props: DiffViewerDialogProps) {
           setMetadataOnly(result.diff !== '' && !contentDiffers);
         }
       })
-      .catch((err) => setError(String(err)))
-      .finally(() => setLoading(false));
+      .catch((err) => {
+        if (thisGen !== fetchGeneration) return;
+        setError(String(err));
+      })
+      .finally(() => {
+        if (thisGen === fetchGeneration) setLoading(false);
+      });
   });
 
   return (

src/components/MonacoDiffEditor.tsx

@@ -60,11 +60,17 @@ export function MonacoDiffEditor(props: MonacoDiffEditorProps) {
   });
 
   createEffect(() => {
-    if (originalModel) originalModel.setValue(props.oldContent);
+    const value = props.oldContent;
+    if (originalModel && originalModel.getValue() !== value) {
+      originalModel.setValue(value);
+    }
   });
 
   createEffect(() => {
-    if (modifiedModel) modifiedModel.setValue(props.newContent);
+    const value = props.newContent;
+    if (modifiedModel && modifiedModel.getValue() !== value) {
+      modifiedModel.setValue(value);
+    }
   });
 
   createEffect(() => {