The Alt key on the right for typing special characters in some languages is called AltGr and is completely different from the normal Alt key. In Windows AltGr is always mapped to Ctrl+Alt
When you press AltGr+A Windows converts it to Ctrl+Alt+A. However it seems Greenshot captures the Ctrl+A before passing the key combination to the keyboard input method which then types ą. Since Ctrl+A has selected all texts, the new key will replace everything with ą. I've confirmed that behavior on my PC:
- In the US layout (no AltGr) pressing Ctrl+Left Alt+A will select all texts just like Ctrl+A
- In another layout with AltGr but AltGr+A has no special character associated with it pressing Ctrl+Left Alt+A will insert A
So it's a bug in Greenshot and can't be fixed without modifying Greenshot source code