Blank windows in some apps in macOS as guest OS in Parallels virtual machine When running Parallels 12.2.0 with macOS Sierra as the guest OS, I see windows that are blank, partially blank, or scaled improperly (zoomed in, magnified) in some apps such as non-Safari web browsers (Firefox, Opera, etc.). The behavior is consistent, either working properly all the time or improperly all the time.
I am guessing that apps handling their own window-drawing rather calling upon Apple's usual drawing libraries is the cause of the bug.
Is there any trick to getting the contents of these apps’ windows to appear correctly?
Bad
Brave 0.18.14 browser

Firefox 54.0.1

Good
In contrast, some apps including some web browsers draw properly.
Safari 10.1.2 browser

Vivaldi 1.6.689.40 browser
I don't think your guess is correct no - there's no requirement to use Apple's drawing libraries for stuff to work in Parallels.

However, it does seem likely that these programs that don't work are 3D-accelerated and you have problems with 3D-acceleration (which is a known problem-area for programs like Parallels). Therefore try disabling 3D acceleration in the Parallels settings for the virtual machine, reboot the virtual machine and your programs should work again.