(core) Allow left pane to auto-expand on mouse over

Summary:
Tweak PagePanels to let the left pane automatically expand on mouse
over. This is to make pages more accessible when the panel is
collapsed.

In this context, when expanding, the left panel overlap the main
content, reducing visual clutter.

Test Plan: updated

Reviewers: jarek

Reviewed By: jarek

Subscribers: anaisconce, jarek

Differential Revision: https://phab.getgrist.com/D3516

app/client/lib/FocusLayer.ts

@@ -186,7 +186,7 @@ export class FocusLayer extends Disposable implements FocusLayerOptions {
  * Because elements getting removed from the DOM don't always trigger 'blur' event, this also
  * uses MutationObserver to watch for the element to get removed from DOM.
  */
-function watchElementForBlur(elem: Element, callback: () => void) {
+export function watchElementForBlur(elem: Element, callback: () => void) {
   const maybeDone = () => {
     if (document.activeElement !== elem) {
       lis.dispose();