Bugzilla

(In reply to Mayank Bansal from comment #14)
> Chrome opening the testcase: 4.4s

(For completeness: Chrome also janks for ~4 seconds when I resize the textbox to make it as narrow as possible.)

(In reply to Jonathan Kew [:jfkthame] from comment #15)
> Curious..... I see much better performance than that in my Nightly on macOS. [...] and it feels pretty responsive. I wonder why your profiles are so different.

I think to trigger badness, you need to *release your mouse button when the textarea is skinny*.  While you're dragging, I think we use interruptible reflow which makes it remain responsive as you keep dragging it. But when you release your mouse button, we do a non-interruptible reflow, which is extremely slow for a skinny textarea.

(In reply to Mayank Bansal from comment #14)
> Chrome opening the testcase: 4.4s

(For completeness: Chrome also janks for ~4 seconds when I resize the textbox to make it as narrow as possible.)

(In reply to Jonathan Kew [:jfkthame] from comment #15)
> Curious..... I see much better performance than that in my Nightly on macOS. [...] and it feels pretty responsive. I wonder why your profiles are so different.

I think to trigger badness, you need to *release your mouse button when the textarea is skinny*.  While you're dragging, I think we use interruptible reflow which makes it remain responsive as you keep dragging it (so we might start an expensive reflow, but we bail out as you resize the textarea to be wider). But when you release your mouse button, we do a non-interruptible reflow, which is extremely slow for a skinny textarea.