If you assign any mouse button to a hotkey the initial press after client start will be ignored.
Here is a video showcasing this exact behavior: 1drv.ms
On earlier reports this was allegedly not reproduceable. Since then I encountered this problem on
- Arch + X11 + Gnome
- Arch + X11 + i3
- Arch + Wayland + Sway
- Manjaro + X11 + Gnome
- Pop!Os + X11 + Gnome
- WSL Ubuntu + whatever they use
- WSL Arch + whatever they use
across multiple machines. So I can not believe that this is just my incompetence.