enhance: keep status check list scrolled on merge box reload (#38597)

Remember the scroll offset when the merge box reloads. This is a crude
patch now, a proper fix would be a fully vue-rendered template with a
json payload from backend, which is much more work. Idiomorph was also
considered but deemed unviable.

web_src/js/features/repo-issue-pull.ts

@@ -53,7 +53,9 @@ function initRepoPullMergeBoxRefresh(el: Element) {
       const resp = await GET(`${pullLink}/merge_box`);
       if (!resp.ok) return;
       const newEl = createElementFromHTML(await resp.text());
+      const scrollTop = el.querySelector<HTMLElement>('.commit-status-list')?.scrollTop;
       el.replaceWith(newEl); // don't morph, do full replacement to make sure data-global-init and Vue components are re-initialized
+      if (scrollTop) newEl.querySelector<HTMLElement>('.commit-status-list')?.scrollTo({top: scrollTop, behavior: 'instant'});
     },
   });
 }