Plugin with native controls hangs Reaper

This is probably fairly unique to my situation, but hoping somebody has run into this as well.

I have a juce audio plugin that doesn’t use juce for the UI. From the main juce plugin editor, I get the native window handle, and then I add my own native controls. If one of my native controls has focus when the plugin editor loses focus, Reaper will hang. Any ideas what could cause this?

This is on Windows 10 with the latest Reaper.