I am also looking for a solution. When I press ALT - "File" menu on top gets triggered and its impossible to work with smoothly with Wacom tablet or mouse, when this happens.
lets say I am using Healing Brush Tool or Clone Brush Tool - they both require CONSTANT ALT key pressing to sample something - and EVERY time top of the "File" menu opens! Also when it is triggered - Undo (Ctrl+Z) does not work and I need to press AGAIN - ALT or ESC key so it exits damn "File" menu!
I tested in Adobe, File menu also gets highlighter but it does NOT bug next key pressing. ALSO I found this video solution with AutoHotkey software and "Alt_menu_acceleration_DISABLER.ahk" macro, file which completely disables ALT menu triggering, BUT it works in all other programs (including adobe), but still does not work in Affinity Photo...
Moved from adobe to affinity (v1.8.5) and this problem is so frustrating, impossible to work smoothly with retouching.
This thread is almost 4 years old, I can not believe there is no solution to fix this.
Maybe it is at least possible to remap ALT to CTRL (or similar key) trough keyboard shortcuts in affinity? Could not find option to this..