Add diff viewer to git commit dialog (#374)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src-web/components/core/Editor/DiffViewer.css

@@ -0,0 +1,39 @@
+.cm-wrapper.cm-multiline .cm-mergeView {
+  @apply h-full w-full overflow-auto pr-0.5;
+
+  .cm-mergeViewEditors {
+    @apply w-full min-h-full;
+  }
+
+  .cm-mergeViewEditor {
+    @apply w-full min-h-full relative;
+
+    .cm-collapsedLines {
+      @apply bg-none bg-surface border border-border py-1 mx-0.5 text-text opacity-80 hover:opacity-100 rounded cursor-default;
+    }
+  }
+
+  .cm-line {
+    @apply pl-1.5;
+  }
+  .cm-changedLine {
+    /* Round top corners only if previous line is not a changed line */
+    &:not(.cm-changedLine + &) {
+      @apply rounded-t;
+    }
+    /* Round bottom corners only if next line is not a changed line */
+    &:not(:has(+ .cm-changedLine)) {
+      @apply rounded-b;
+    }
+  }
+
+  /* Let content grow and disable individual scrolling for sync */
+  .cm-editor {
+    @apply h-auto relative !important;
+    position: relative !important;
+  }
+
+  .cm-scroller {
+    @apply overflow-visible !important;
+  }
+}

src-web/components/core/Editor/DiffViewer.tsx

@@ -0,0 +1,64 @@
+import { yaml } from '@codemirror/lang-yaml';
+import { syntaxHighlighting } from '@codemirror/language';
+import { MergeView } from '@codemirror/merge';
+import { EditorView } from '@codemirror/view';
+import classNames from 'classnames';
+import { useEffect, useRef } from 'react';
+import './DiffViewer.css';
+import { readonlyExtensions, syntaxHighlightStyle } from './extensions';
+
+interface Props {
+  /** Original/previous version (left side) */
+  original: string;
+  /** Modified/current version (right side) */
+  modified: string;
+  className?: string;
+}
+
+export function DiffViewer({ original, modified, className }: Props) {
+  const containerRef = useRef<HTMLDivElement>(null);
+  const viewRef = useRef<MergeView | null>(null);
+
+  useEffect(() => {
+    if (!containerRef.current) return;
+
+    // Clean up previous instance
+    viewRef.current?.destroy();
+
+    const sharedExtensions = [
+      yaml(),
+      syntaxHighlighting(syntaxHighlightStyle),
+      ...readonlyExtensions,
+      EditorView.lineWrapping,
+    ];
+
+    viewRef.current = new MergeView({
+      a: {
+        doc: original,
+        extensions: sharedExtensions,
+      },
+      b: {
+        doc: modified,
+        extensions: sharedExtensions,
+      },
+      parent: containerRef.current,
+      collapseUnchanged: { margin: 2, minSize: 3 },
+      highlightChanges: false,
+      gutter: true,
+      orientation: 'a-b',
+      revertControls: undefined,
+    });
+
+    return () => {
+      viewRef.current?.destroy();
+      viewRef.current = null;
+    };
+  }, [original, modified]);
+
+  return (
+    <div
+      ref={containerRef}
+      className={classNames('cm-wrapper cm-multiline h-full w-full', className)}
+    />
+  );
+}