From da90af78a1d0706f17391027953ae10c36863ebe Mon Sep 17 00:00:00 2001
From: Je Xia <i@jex.me>
Date: Mon, 20 Jul 2026 23:43:18 +0800
Subject: [PATCH] [diffs/edit] Clamp selection rendering to render window

---
 packages/diffs/src/editor/editor.ts           |  9 ++++-
 .../diffs/test/editorVirtualizedEdit.test.ts  | 38 ++++++++++++++++++-
 2 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/packages/diffs/src/editor/editor.ts b/packages/diffs/src/editor/editor.ts
index 20e3cbd1c..034eaa8df 100644
--- a/packages/diffs/src/editor/editor.ts
+++ b/packages/diffs/src/editor/editor.ts
@@ -3737,7 +3737,14 @@ export class Editor<LAnnotation> implements DiffsEditor<LAnnotation> {
     }
 
     const { start, end } = range;
-    for (let line = start.line; line <= end.line; line++) {
+    let firstLine = start.line;
+    let endLine = end.line + 1;
+    if (this.#renderRange !== undefined) {
+      const { startingLine, totalLines } = this.#renderRange;
+      firstLine = Math.max(firstLine, startingLine);
+      endLine = Math.min(endLine, startingLine + totalLines);
+    }
+    for (let line = firstLine; line < endLine; line++) {
       if (!this.#isLineVisible(line)) {
         continue;
       }
diff --git a/packages/diffs/test/editorVirtualizedEdit.test.ts b/packages/diffs/test/editorVirtualizedEdit.test.ts
index 532ad57e5..56fa8361f 100644
--- a/packages/diffs/test/editorVirtualizedEdit.test.ts
+++ b/packages/diffs/test/editorVirtualizedEdit.test.ts
@@ -1,8 +1,9 @@
-import { afterAll, describe, expect, test } from 'bun:test';
+import { afterAll, describe, expect, spyOn, test } from 'bun:test';
 
 import { File } from '../src/components/File';
 import { DEFAULT_THEMES } from '../src/constants';
 import { Editor } from '../src/editor/editor';
+import { DirectionForward } from '../src/editor/selection';
 import { disposeHighlighter } from '../src/highlighter/shared_highlighter';
 import type { FileContents, RenderRange } from '../src/types';
 import { installDom, wait } from './domHarness';
@@ -304,3 +305,38 @@ describe('Editor edits at the bottom of a virtualized window', () => {
     }
   });
 });
+
+describe('Editor selections in a virtualized window', () => {
+  test('limits a document-spanning selection to the rendered lines', async () => {
+    const range = makeRange(4900, 7);
+    const { cleanup, content, editor, fileContainer } =
+      await createWindowedEditor(10_000, range);
+    editor.setOptions({ roundedSelection: false });
+
+    const querySelector = spyOn(content, 'querySelector');
+    try {
+      editor.setState({
+        selections: [
+          {
+            start: { line: 0, character: 0 },
+            end: { line: 9999, character: 10 },
+            direction: DirectionForward,
+          },
+        ],
+        view: { scrollLeft: 0, scrollTop: 0 },
+      });
+
+      expect(
+        querySelector.mock.calls
+          .map(([selector]) => /^\[data-line="(\d+)"\]/.exec(selector)?.[1])
+          .filter((line) => line !== undefined)
+      ).toEqual(['10000']);
+      expect(
+        fileContainer.shadowRoot?.querySelectorAll('[data-selection-range]')
+      ).toHaveLength(range.totalLines);
+    } finally {
+      querySelector.mockRestore();
+      cleanup();
+    }
+  });
+});