User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:93.0) Gecko/20100101 Firefox/93.0

Steps to reproduce:

1. Go to a website with text (happens on all websites).
2. Zoom using trackpad and not from zoom controls.
3. Scroll up and down normally

Actual results:

The text shifts up and down (see attached video). Its really noticeable. This issue is not present on either Chrome or Safari.

Expected results:

Text should not jump up and down.

The Bugbug bot thinks this bug should belong to the 'Core::Panning and Zooming' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

Such a bug has existed forever, but I think it became worse sometime between June and August.

The severity field is not set for this bug.
:jimm, could you have a look please?

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An extreme example on a normal website.

This bug happens on Firefox for Android as well. It is especially noticeable when using Desktop version of websites.

A more subtle example is how the text changes size on the spinning button at top left on the website https://projects.verou.me/prefixfree/ - text rendering when in an animated state seems to have different snapping?

Fixing that type of problem in general for all types of animations is probably hard. Scrolling is a very common case where we should be able to avoid it though.

Another example is visible in the bug https://bugzilla.mozilla.org/show_bug.cgi?id=1782405 where the test case https://codepen.io/konstantindenerz/full/oNNaEVp when animating looks like about a half-pixel sampling offset is applied to filters compared to not animating?

Edit: seems like a different issue from the scrolling, nevermind.

Emilio's test case in bug 1801098#c17 is quite nice. I do easily see this bug on the zoomed test case by dragging the vertical scrollbar (with a local m-c build with pixel snapping).