Attaching file is an example to see the issue, to reproduce the issue;

1. Open the attachment
2. Pinch zoom in as possible
3. Scroll to right bottom of the content
4. Try to scroll upward slowly, thus you can see the dynamic toolbar at bottom, but a gray position:fixed element will be covered by the toolbar instead of moving along with the bar.

I think we should use GetCurrentAsyncTransform() instead of GetPresShellResolution() but I haven't found a way to obtain a correct value there. Anyways, with this patch the position:fixed element moved much better, but still looks something wrong.

(In reply to Hiroyuki Ikezoe (:hiro) from comment #0)

4. Try to scroll upward slowly, thus you can see the dynamic toolbar at bottom, but a gray position:fixed element will be covered by the toolbar instead of moving along with the bar.

Why is "moving along with the bar" the desired behaviour?

When we are zoomed in, the visual viewport is smaller than the layout viewport. Fixed elements are attached to the layout viewport. When you scroll up, the visual viewport's bottom edge moves away from the layout viewport's bottom edge, so we expect the fixed element to scroll out of view.

Hmm, that's fair enough. So maybe we need to do somewhat different in the case where the visual viewport differs from the layout viewport there?

Also, to be fair, the behavior on Chrome looks also broken in a different way.

It's not entirely clear to me what the desired behaviour is, but in any case this seems like an edge case interaction between zooming and the dynamic toolbar.