Resize map on div size change

By default, OpenLayers listens for and resizes on page resize, but not
on map div size change. (Pre-2021ish, this was maybe impossible to do?
But ResizeObserver makes this easy.) This means from the first time the
sidebar is collapsed until the first time a resize event was triggered
(e.g. on mobile rotating the device) the map wouldn't render render its
full size.

main.js

@@ -118,3 +118,5 @@ map.on('click', function (evt) {
 
   popupOverlay.setPosition(evt.coordinate);
 });
+
+new ResizeObserver(() => map.updateSize()).observe(document.getElementById("map"));