I tested your GainPlugin on Linux and I’m also experiencing problems with a sluggish UI and missing redraws. This includes the “Options” popup window provided by JUCE. This is for the Standalone version of your plugin, for some reason Bitwig on Linux has failed to load the VST3 version so far.
This sounds like it could be the same issue that I posted here
The application also uses mixed 2D / GL accelerated rendering.
In that thread I posted a minimal example project to reproduce the problem as well. It is also built via CMake but assumes that JUCE has been deployed somewhere (using the INSTALL target or make install respectively). CMake has to be told where to find the JUCE installation by using the JUCE_DIR environment variable to point to the .cmake files (in lib/cmake/JUCE-X.Y.Z relative to the installation root).
It would be great if someone could test this on Windows 11 and report whether the same issue occurs. I only have Windows 10 to test here, where the issue was not present. Maybe it is not Linux- or Win11-specific after all but a more general issue.
Best,
Patric