Can't really give a way to reproduce this, but sometimes, when switching between fullscreen apps, MacVim has a seizure.

Happened to me three times, and the pattern of the "bug" is not the same. When I took the screenshot, every transparent pixel was black. I have no idea why.
It's not a really important bug, as it disappears as soon as I click on the screen.
Also have experienced this, seems to happen when the native full screen feature is enabled.
Same issue for me, when the native full screen feature is on. macOS High Sierra.
There are suggestions for workarounds in #557
Workarounds either don't work or slow down MacVim :(
This is a pretty major problem for me. I'm using an external monitor, and have tried turning off Core Renderer as well as native full screen. Neither really fix the problem or introduce other issues that are just as bad. Are the authors able to address whats going on here?
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Also have experienced this, seems to happen when the native full screen feature is enabled.