Fixes collapsed files not loading in single file mode

Changelog: fixed

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/290745

app/assets/javascripts/diffs/components/diff_file.vue

@@ -205,6 +205,8 @@ export default {
 
     if (this.hasDiff) {
       this.postRender();
+    } else if (this.viewDiffsFileByFile && !this.isCollapsed) {
+      this.requestDiff();
     }
 
     this.manageViewedEffects();

spec/frontend/diffs/components/diff_file_spec.js

@@ -521,4 +521,24 @@ describe('DiffFile', () => {
       expect(button.attributes('href')).toBe('/file/view/path');
     });
   });
+
+  it('loads collapsed file on mounted when single file mode is enabled', async () => {
+    wrapper.destroy();
+
+    const file = {
+      ...getReadableFile(),
+      load_collapsed_diff_url: '/diff_for_path',
+      highlighted_diff_lines: [],
+      parallel_diff_lines: [],
+      viewer: { name: 'collapsed', automaticallyCollapsed: true },
+    };
+
+    axiosMock.onGet(file.load_collapsed_diff_url).reply(httpStatus.OK, getReadableFile());
+
+    ({ wrapper, store } = createComponent({ file, props: { viewDiffsFileByFile: true } }));
+
+    await wrapper.vm.$nextTick();
+
+    expect(findLoader(wrapper).exists()).toBe(true);
+  });
 });