[6.x] Fix sidebar not showing after quick resize

[duncanmcclean]

This pull request fixes an issue where resizing a publish form quickly could lead to the sidebar not being "expanded" correctly.

The ElementContainer.vue used in Publish/Tabs.vue uses a throttled ResizeObserver, but the throttle would only fire once immediately, then ignore all calls for 200ms. If resizing finished within that window, the final width was never reported.

Fixes #13915

[jasonvarga]

AI pointed out a harmless issue, but I've resolved it anyway.

No cleanup/cancellation mechanism. The ElementContainer creates a ResizeObserver in mounted but never disconnects it or cancels pending timeouts on unmount. This is a pre-existing
issue (not introduced by this PR), but the trailing setTimeout does make it slightly more relevant — a timeout could fire after the component is destroyed. That said, it would just
set this.width on a destroyed component, which is harmless in practice. Not a blocker.

I also added tests.