Hidden keep-alive pane layers stay mounted with visibility:hidden and
pointer-events:none so scroll position and state survive tab round-trips.
On some compositors these hidden layers can still paint for one frame during
a tab switch, causing accent-colored elements from the outgoing pane to flash
over the incoming pane (e.g. Notes ↔ Tokens).
Add opacity-0 so the compositor drops the hidden layer entirely while keeping
the DOM mounted. This fixes the flash without losing scroll/state preservation.
Closes#101476
b592c82892