Secondary side bar becomes less compact and loses the close button when moving activity bar to the top

[Tyriar]

When activity bar is default:

When activity bar is on the top it's less compact and there's no close button(!):

I tried activity bar on the top to make things more compact but they ended up having the opposite effect for the unrelated secondary side bar. Note that I also hide the layout controls to avoid visual noise so I rely upon the close button.

[gjsjohnmurray]

See #232719 about the Close button.