This happens in Sourcetree 3.4.12 and previous versions.
If I select one or more lines from any committed source file and click CTRL+C, then I switch focus to any other application (Notepad for example) - Sourcetree freezes for 10-15 seconds and during this time if I press CTRL+V - nothing is pasted but after Sourcetree unfreezes - CTRL+V works and text from Sourcetree is copied into Notepad.
This bug is counter-productive - I need to wait 20 seconds for each copy/paste operation.
I have Windows 10 Pro.