When opening the JUCE demo plugin (latest tip) in REAPER (5.28, on what I had before I updated REAPER it worked ok) on macOS 10.12, it looks like this:
or in Reaper’s FX window like this:
It looks ok after a horizontal resize (via Reaper’s resize handles) but vertical resizes via REAPER’s handle or the demo plugin’s handle messes it up again. After closing the plugin window and opening it again it starts messed up again.
Just curious, what happens if you un-dock the VST from the FX-bay?
I’d be happy to check that, but REAPER isn’t my main DAW so I’m unaware what this undocking thing means… Please explain to me like I’m 5 and I’ll happily check it 
Oh sorry, double click on the VST’s name in the column ( highlighted blue above ), and your plugin will pop out, into standalone mode.
@Ian: That’s already shown in the first screenshot.
Btw. I can reproduce it here on OS X 10.11.6, VST3 looks exactly the same as in the screenshots by @yairadix.
Doh, I thought that was a previous effort. Never mind.
A fix for this is on develop now.
Awesome, thanks for the quick response! Works well and also on REAPER’s newly released 5.29
