[Bug 1940417]

[Jani Uusitalo]

I can't speak for Jeroen, but I played around with the upstream builds
(both 92 and 91) and my faulty profile a bit and managed to isolate the
trigger combination for this: `MOZ_ENABLE_WAYLAND=1` and setting
`layout.css.devPixelsPerPx` to my preferred value of `1.2`; either
resetting `layout.css.devPixelsPerPx` to its default (`-1.0`), or
running Firefox without `MOZ_ENABLE_WAYLAND=1` will make the toolbar
items work again. [Here's the graphics portion of my
`about:support`](https://pastebin.mozilla.org/OdgM9Oox).

**So my steps to reproduce this are:**
1. start Firefox with `MOZ_ENABLE_WAYLAND=1`
2. set `layout.css.devPixelsPerPx` to `1.2`
3. click on the hamburger menu button

**What I expect to happen:**
For the hamburger menu content to show.

**What happens instead:**
The hamburger menu content flickers or doesn't show at all.

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https://bugs.launchpad.net/bugs/1940417

Title:
  On Wayland toolbar menus are sometimes invisible/flickering

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