Allow panel to be scrolled again

[nikolas]

This is a quick fix for a major bug: the object list is currently not scrollable.

Closes #586

Re-introduce overflow-y: auto so panel can be scrolled when there are many objects.

Also, this bug highlighted an issue with my new ResizeObserver setup - this observer needs to be disabled when the pointermove event is updating the width.

[Evan-CTL]

When there is overflow and the scrollbar appears the drag button constantly fights to return the panel to its default size. Making it basically unusable when the object list overflows past the browser window.

Side Note: Should the drag button be hidden when the menu is collapsed/hidden?

[nikolas]

Okay, it turns out the ResizeObserver I introduced was completely unnecessary, and just causing problems. I'm not sure why I thought I needed this in the first place, looking back.

Anyways, try one more time on this updated branch. The resizer now appears to be working properly when the scroll bar is present.

[nikolas]

And yeah, the drag button doesn't do anything when the panel is hidden, so we can just remove it or otherwise disable it when it's in that state. I'll make those changes on a separate PR, as there's some subtlety around when to hide it, to make it not look glitchy.