Affected versions

• 84.0.2 (20201221093813)
• 85.0b5 (20210105185604)
• 86.0a1 (20210107040715)

Affected platforms

• Ubuntu 18.04

Steps to reproduce

1. Open Firefox and resize the browser horizontally making it smaller.
2. Observe the top right browser buttons.

Expected result

• Buttons are displayed as expected.

Actual result

• Buttons are shaking.

Regression range

• Last good revision: 9982d52f7ba861051b9ac7c3b2ed1465985a38be (2019-04-09)
• First bad revision: 30ca3c3abfe63d2d5923e28dcc565972ea4cf811 (2019-04-10)
• Pushlog:
  https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=9982d52f7ba861051b9ac7c3b2ed1465985a38be&tochange=30ca3c3abfe63d2d5923e28dcc565972ea4cf811
  Possible regressor: bug 1543217
  Note: On older builds before the regressor date, I can see something similar with this bug as well but only after I am dragging the browser window, and is not so obvious.

Notes

• Attached a screen recording.
• I tried with gfx.webrender.force-disabled:true on 84.0.2. At first, I cannot reproduce the issue but after the window is dragged to another point a few times the issue is starting to reproduce. The difference is that is not so obvious with WR off.
• If Title Bar is checked I cannot reproduce the issue.