There is almost surely an issue when
docking and un-docking the ME window, but it seems of larger scope than only this.
No matter what monitor I reproduce the steps in, I get either an unclickable blank window with a titlebar or a smaller child window within a parent window at some point. It can affect only the ME window when I am executing the steps on the 2nd monitor.
But when I am executing the dock/undock steps with the ME window being on my main monitor, it will break the Reaper main window too.
Seems to be sometimes random but the pattern is very similar each time.
In the worse case scenario,
I downloaded rc3 from LoL, installed x64 portable in new folder then openned Reaper for first time.
Then maximized the main window and it broke already.
It happens on fresh portable installations too.
I've made sure there is no DPI complexities involved.
I'm running win10 latest version with most recent nvidia graphics driver.
Try some docking, un-docking, closing and re-openning the MIDI editor window and there might come the issue.
Hope this can help and thanks for your time
Examples: