Keeping the main window toolbar activated while the modal overlay is shown could create the appearance of the non-responsive GUI.
Fixes #22.
On master (ca055885c631de8ac0ffe24be6b02835dbcc039d):

With this PR:

Tested ACK d0cc1f6df740e03ca0213a3754c3277b01ae2c05. Tested on Linux/X11 as much as I could given it's a pretty small change; seems like a nice improvement. I'm not experienced in Qt, but I don't see anything obviously problematic about the code.
Tested ACK. Tested on Ubuntu 20.04 LTS. It can improve user experience.
ACK d0cc1f6 tested on Debian 5.7.6-1 (2020-06-24) x86_64 GNU/Linux
Might need a macOS and Windows reviewer.
ACK d0cc1f6df740e03ca0213a3754c3277b01ae2c05 tested on Ubuntu 18.04.4 LTS
I didn't review the code but verified the behavior with and without the PR, and that src/qt/test/test_bitcoin-qt passes.
@polylunar thanks! It is very helpful to review before or after a merge. Great to see and looking forward to more tests and review from you :rocket: