Bugzilla

Here's a profile I just captured to demonstrate this being responsive and also hanging: https://share.firefox.dev/3Axh8YO
In the profile:
- For the first half, I shrink+grow the textarea (as skinny as possible and then back to wide again), releasing my mousebutton when it's wide.
- For the last half, I shrink the textarea, releasing my mousebutton when it's skinny.
- The final `PresShell::DoFlushPendingNotifications Layout` operation (when I release my mousebutton) is 216ms long when the textarea is wide, vs. 16s long when it's skinny.  (Note that I didn't quite make it as skinny as possible; it would be worse if I did that.)
- All the other reflows are `PresShell::DoFlushPendingNotifications InterruptibleLayout` to keep things responsive during my drag operation. (So they might be theoretically slow reflows, but we're able to bail out of them.)

Here's a profile I just captured to demonstrate this being responsive in part of the profile and also hanging in another part of the profile:
https://share.firefox.dev/3Axh8YO

In this profile:
- For the first half, I shrink+grow the textarea (as skinny as possible and then back to wide again), releasing my mousebutton when it's wide.
- For the last half, I shrink the textarea, releasing my mousebutton when it's skinny.
- The final `PresShell::DoFlushPendingNotifications Layout` operation (when I release my mousebutton) is 216ms long when the textarea is wide, vs. 16s long when it's skinny.  (Note that I didn't quite make it as skinny as possible; it would be worse if I did that.)
- All the other reflows are `PresShell::DoFlushPendingNotifications InterruptibleLayout` to keep things responsive during my drag operation. (So they might be theoretically slow reflows, but we're able to bail out of them.)

Here's a profile I just captured to demonstrate this being responsive in part of the profile and also hanging in another part of the profile:
https://share.firefox.dev/3Axh8YO

In this profile:
- For the first half, I shrink+grow the textarea (as skinny as possible and then back to wide again), releasing my mousebutton when it's wide.
- For the last half, I shrink the textarea, releasing my mousebutton when it's skinny.
- There's a final `PresShell::DoFlushPendingNotifications Layout` operation (when I release my mousebutton) at the end of each of these resize operations; it's only 216ms long when I leave the textarea in a wide state, vs. it's 16s long when I leave the textarea in a skinny state.  (Note that I didn't quite make it as skinny as possible; it would be worse if I did that.)
- All the other reflows are `PresShell::DoFlushPendingNotifications InterruptibleLayout` to keep things responsive during my drag operation. (So they might be theoretically slow reflows, but we're able to bail out of them.)