This is a common problem related to the GTK2 issues on non-GTK hosts. The behaviour of UI is unpredictable: UI can not react on mouse/keyboard events, utilize a lot of CPU, do not render or even freeze.
To avoid these problems, currently a lot of work is done for porting all widgets to plain X11 protocol implementation (xlib).
Currently known affected hosts: ANY JUCE-based host, Renoise, Reaper.
Hosts that are not affected: Ardour, Mixbus, Carla.
As a workaround solution currently can be used this:
https://github.com/osxmidi/LinVst/tree/linux
This solution was promoted by user osxmidi in the thread on Reaper's forum:
https://forum.cockos.com/showpost.php?p=1892962&postcount=34
The thread about UI freezes on Renoise forum:
http://forum.renoise.com/index.php/topic/49629-linux-studio-pluginslsp-graphics-freeze/
And the thread on LinuxMusicians:
https://linuxmusicians.com/viewtopic.php?f=48&t=17012&hilit=lsp#p80545
This ticket is created because there is many feedback received about non-working plugins. When the UI problem will be fixed, this ticket will be closed.
Closing this issue because the UI has been moved to X11 in the new UI.