diff --git a/web_src/js/features/repo-diff-selection.test.ts b/web_src/js/features/repo-diff-selection.test.ts
new file mode 100644
index 0000000000..c2b035e978
--- /dev/null
+++ b/web_src/js/features/repo-diff-selection.test.ts
@@ -0,0 +1,86 @@
+import {applyDiffLineSelection} from './repo-diff-selection.ts';
+
+function createDiffRow(tbody: HTMLTableSectionElement, options: {id?: string, lineType?: string} = {}) {
+  const tr = document.createElement('tr');
+  if (options.lineType) tr.setAttribute('data-line-type', options.lineType);
+
+  const numberCell = document.createElement('td');
+  numberCell.classList.add('lines-num');
+  const span = document.createElement('span');
+  if (options.id) span.id = options.id;
+  numberCell.append(span);
+  tr.append(numberCell);
+
+  tr.append(document.createElement('td'));
+  tbody.append(tr);
+  return tr;
+}
+
+describe('applyDiffLineSelection', () => {
+  beforeEach(() => {
+    document.body.innerHTML = '';
+  });
+
+  test('selects contiguous diff rows, skips expansion rows, and clears previous selection', () => {
+    const fragment = 'diff-selection';
+
+    const otherBox = document.createElement('div');
+    const otherTable = document.createElement('table');
+    otherTable.classList.add('code-diff');
+    const otherTbody = document.createElement('tbody');
+    const staleActiveRow = document.createElement('tr');
+    staleActiveRow.classList.add('active');
+    otherTbody.append(staleActiveRow);
+    otherTable.append(otherTbody);
+    otherBox.append(otherTable);
+
+    const container = document.createElement('div');
+    container.classList.add('diff-file-box');
+    const table = document.createElement('table');
+    table.classList.add('code-diff');
+    const tbody = document.createElement('tbody');
+    table.append(tbody);
+    container.append(table);
+
+    const rows = [
+      createDiffRow(tbody, {id: `${fragment}L1`}),
+      createDiffRow(tbody),
+      createDiffRow(tbody, {lineType: '4'}),
+      createDiffRow(tbody),
+      createDiffRow(tbody, {id: `${fragment}R5`}),
+      createDiffRow(tbody),
+    ];
+
+    document.body.append(otherBox, container);
+
+    const range = {fragment, startSide: 'L' as const, startLine: 1, endSide: 'R' as const, endLine: 5};
+    const applied = applyDiffLineSelection(container, range);
+
+    expect(applied).toBe(true);
+    expect(rows[0].classList.contains('active')).toBe(true);
+    expect(rows[1].classList.contains('active')).toBe(true);
+    expect(rows[2].classList.contains('active')).toBe(false);
+    expect(rows[3].classList.contains('active')).toBe(true);
+    expect(rows[4].classList.contains('active')).toBe(true);
+    expect(rows[5].classList.contains('active')).toBe(false);
+    expect(staleActiveRow.classList.contains('active')).toBe(false);
+  });
+
+  test('returns false when either anchor is missing', () => {
+    const fragment = 'diff-missing';
+    const container = document.createElement('div');
+    container.classList.add('diff-file-box');
+    const table = document.createElement('table');
+    table.classList.add('code-diff');
+    const tbody = document.createElement('tbody');
+    table.append(tbody);
+    container.append(table);
+    document.body.append(container);
+
+    createDiffRow(tbody, {id: `${fragment}L1`});
+
+    const applied = applyDiffLineSelection(container, {fragment, startSide: 'L' as const, startLine: 1, endSide: 'R' as const, endLine: 2});
+    expect(applied).toBe(false);
+    expect(container.querySelectorAll('tr.active').length).toBe(0);
+  });
+});
diff --git a/web_src/js/features/repo-diff-selection.ts b/web_src/js/features/repo-diff-selection.ts
index 60fadc86e2..bc00f04fdf 100644
--- a/web_src/js/features/repo-diff-selection.ts
+++ b/web_src/js/features/repo-diff-selection.ts
@@ -36,7 +36,7 @@ function parseDiffAnchor(anchor: string): DiffAnchorInfo | null {
   return {anchor, fragment, side, line};
 }
 
-function applyDiffLineSelection(container: HTMLElement, range: DiffSelectionRange): boolean {
+export function applyDiffLineSelection(container: HTMLElement, range: DiffSelectionRange): boolean {
   // Find the start and end anchor elements
   const startId = `${range.fragment}${range.startSide}${range.startLine}`;
   const endId = `${range.fragment}${range.endSide}${range.endLine}`;