Totally reproducible:

• create empty repository
• commit a file in mater
• create a new branch
• commit a file there
• merge branch into master

The application freezes, that is it does not accept any mouse/keyboard commands anymore. The menus are mostly greyed out.

As far as I can tell, the operation itself is performed.

But due the application freezing, it's not possible to work with it anymore and I need to Forge Quit it.