Closes #33972

As noted on
https://github.com/zed-industries/zed/pull/31390#discussion_r2147473526,
when splitting panes and having a border size set for the active pane,
or the minimap visibility configured to the active editor only, zed will
shortly show a flicker of the border or the minimap on the pane that's
being deactivated.

Release Notes:

- Fixed an issue where pane activations would sometimes have a brief
delay, causing a flicker in the process.