Bugzilla

For good measure, I tried the STR with 3000 tabs (up from 1000 in comment 0) -- here's a profile under that configuration:

https://share.firefox.dev/3J9cY8K

Observations:
- With the additional tabs, the long-restyle on enter/exit fullscreen does get proportionally longer (3000 tabs incurs a ~240ms restyle, around 2.5x the duration of the restyle that we incur for 1000 tabs).
- ...and we do spend a substantial amount of time in a "red jank bar" state -- about 1.7s when entering full screen and 1.4s when exiting
- ...and a substantial portion of that seems to be dispatching DOM events (presumably to each tab), with the DOM event called "underflow" on entering fullscreen (~330ms spent spent dispatching those) and "overflow" on exiting fullscreen (~280ms spent dispatching those).

As an additional test: I tried the STR with 3000 tabs (up from 1000 in comment 0), since the original reporter in bug 1733227 had >2000 tabs and I wanted to overshoot that a bit for good measure.

Here's a profile under that configuration:
https://share.firefox.dev/3J9cY8K

Observations:
- With the additional tabs, the long-restyle on enter/exit fullscreen does get proportionally longer (3000 tabs incurs a ~240ms restyle, around 2.5x the duration of the restyle that we incur for 1000 tabs).
- ...and we do spend a substantial amount of time in a "red jank bar" state -- about 1.7s when entering full screen and 1.4s when exiting
- ...and a substantial portion of that seems to be dispatching DOM events (presumably to each tab), with the DOM event called "underflow" on entering fullscreen (~330ms spent spent dispatching those) and "overflow" on exiting fullscreen (~280ms spent dispatching those).