STR: 1. Open some image directly within Firefox. 2. Zoom in. 3. Scroll up and down, so the URL bar hides/unhides. Actual: After the URL bar has become completely hidden/unhidden, the image makes a jump approximately the height of the URL bar in the opposite direction (i.e. if I'm scrolling towards the top of the picture, the viewport jumps back down and vice versa). The jump normally happens as soon as the URL bar has finished animating, although once it only occured after also lifting the finger. Expected: No jumping around. Note: Images are the most prominent example, but I can also reproduce this with the media controls that are displayed when directly opening e.g. an MP3 file. Normal web pages seem unaffected though. I can also reproduce this with and without APZ in all current Firefox versions.
Sample URL on which I can repro this.
FWIW I suspect this is because the toolbar show/hide resizes the height available to content, and the ImageDocument implementation probably uses that to position the image.