Rendering issues since 26.2 on Mojave

[tgvaughan]

This is a relatively minor issue, but since upgrading (via brew cask) to 26.2 from 26.1 I'm seeing some strange display refresh issues on my dark-themed Mojave system. Examples include:

• button mouse-over hilighting not being cleared when the pointer moves away from the button,
• areas of the display being left blank after a page-down or page-up.

Unfortunately I haven't yet found a way to systematically reproduce this, but it definitely became an issue only after upgrading (i.e. I've never seen it before).

[tgvaughan]

In retrospect this is probably an upstream problem.