User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:69.0) Gecko/20100101 Firefox/69.0

Steps to reproduce:

Just browsing.

Actual results:

From time to time FireFox shrinks text vertically, skipping a few lines, during scrolling -- exactly at the very threshold between visible and not yet visible text if the users make a pause during the scrolling. The issue stays on until the user scrolls the part completely away up or down so the corrupted part would be re-rendered, and then it looks perfectly fine.

Expected results:

Perfectly accurate text at all times.

Can you check the "Compositing" feature in about:support? Does it say "WebRender"?

Yes.

I forgot to add information that might be relevant: as you can see from the screenshot, my Windows 10 runs in graphics scaling mode (about 138% of versus 100% scale by default). This should not affect the issue of lines being skipped at the border of the "viewport" after scrolling is paused, but still.

So it looks like a couple of lines get either erased or duplicated at the border of the scrolling viewport.

I suspect this is a dup of bug 1565473

Please leave a note or reopen this bug in case you see it again. I saw it on Reddit, but bug 1566712 fixed it.