922918 - Very bad performance on CSS transform demo

Status: RESOLVED DUPLICATE of bug 926706

(Core :: Layout, defect)

Description

[David Anderson [:dvander] - inactive, e-mail if emergency]

The demo at the attached URL is extremely janky on my Linux desktop. I'm guessing I'm getting about 5-10fps. It looks flawlessly smooth in Chrome. I do have acceleration force-enabled and my about:support says OGL.

On Windows, I fare a little better, but it's still very janky. A profile shows most of the time is in painting. With Chrome's builtin profiler it looks like it's just recomputing the style and recompositing. It doesn't paint unless I reload the page.

Matt Woodrow took a preliminary look at this and found what might be the cause: one of the elements is overflowing the page, which triggers reflow of the entire document on every frame. It's a no-op reflow, but the text items get invalidated anyway, so we have to repaint. He guessed that we need some way to detect no-op reflows and skip invalidating those elements.