User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:95.0) Gecko/20100101 Firefox/95.0

Steps to reproduce:

I zoomed-in part of a web page using smooth zoom (<- -> gesture on the touchpad under Windows or on the screen under Android ) a lot so the text lines look big, and scrolled the text lines slowly (two-finger swipe up under Windows or swipe up under Android).

Under Windows, it happens on a lot of sites (for example, Wikipedia). Under Android, the comment section of mobile YouTube site.

Under Windows, you need to scroll down quite a bit (probably until the portion that was not visible before the zoom-in begin to move into the view) in order to reproduce the problem. You need to scroll it very slowly.

Actual results:

The text lines do not move at the same pace, so the lines look shaking up and down.

Expected results:

Text lines move at the same pace.

Max, thanks for this bug report! IIUC what you want, the same pace you mean is same pixels? Then I guess bug 1720084 is same as you want?

I think what the reporter is describing here is that the vertical positions of text lines are unstable when zoomed in. I see a similar effect on macOS as well; it's easier to reproduce, in my experience, by slightly resizing the window.

This does not occur if the old full-page-zoom is used to make the content similarly large; it seems to specifically be a result of using pinch-zoom.

To clarify, I'll attach a screen recording that illustrates the effect I'm seeing.

This shows a Wikipedia page in Nightly on macOS, with pinch-zoom having been used to zoom in greatly; then the window size is adjusted slightly back and forth. Observe how the text lines seem to wobble up/down at times, so that their spacing is irregular.

Similar erratic vertical placement of the lines sometimes occurs when scrolling, but seems harder to catch it in the act.

(I suspect at least part of what's going on here is that layout is trying to snap line positions to integer device pixels, but is unaware of the zoom factor in effect, and so the snapping is happening in terms of "unzoomed" pixels and is far too coarse for the zoomed-in view.)

The problem of pinch zooming in and then scrolling and having lines up text move slightly up or down is something I see frequently, and I believe it is a problem that is limited to webrender. For example bug 1736757 has an example where it happens to both text and images, it's much more noticeable on the images though.

The resize problem could be that, or it could be something in layout. Can you reproduce the resize problem in an older build of Firefox where you can disable webrender?

This does seem to be webrender-specific; running an older build with WR disabled, I can't reproduce it.

Most likely this is the same underlying issue as your bug 1736757 example. Marking as see-also, but feel free to dupe it if you're confident it is the same thing.

Let's mark it as dependent; once bug 1736757 is fixed, we can re-test this and confirm that it was the same issue.

(In reply to Hiroyuki Ikezoe (:hiro) from comment #1)

Max, thanks for this bug report! IIUC what you want, the same pace you mean is same pixels? Then I guess bug 1720084 is same as you want?

No, it is a different issue. I did not mean that the scrolling speed is uneven; I meant that each line seem to move slightly different speed. For example, in the case of "scrolling speed is uneven", if my fingers are moving at constant speed, it will be something like:

• At 0.001s, line 1 is at 10px, line 2 is 20px
• At 0.002s, line 1 is at 14px, line 2 is 24px
• At 0.003s, line 1 is at 20px, line 2 is 30px

but in my case, it would be something like:

• At 0.001s, line 1 is at 10px, line 2 is 20px
• At 0.002s, line 1 is at 15px, line 2 is 24px
• At 0.003s, line 1 is at 20px, line 2 is 31px

I thought that people would understand, but I guess I will have to attach some sort of screen recording. I will add some below after making them.

This is Android FireFox (96.3.1) at the comment section of the mobile YouTube site, but the phenomenon is the same with the desktop FireFox.

I am going to make this bug as a dup of bug 1735008, bug 1735008 has been tracked by a blocker of the wr-snap meta bug (bug 1487858).