`ThreadView::regenerate` does `thread.rewind()` which de-focused the
message editor mid-flight causing the zoomed panel to disappear.

* workspace: Keep zoomed panels open when window focus is lost

Commit adds a test + tweaks a default, generic, fallback to be more
lenient: instead of focusing the workspace (and hiding all panels right
away), try to fall back to whatever panel open

* gpui: Restore focus to nearest surviving ancestor when focus is lost

Commit tries to push it down to a library level and add a way to fall
back to closest ancestor that's still visible with the focus instead.


Release Notes:

- Fixed the zoomed agent panel closing unexpectedly on rewind