When you press single Alt button or in combination with another key i always see this menu:
How to prevent this behavior only for Opera browser in GNOME? Because some similar behavior have another programs like Firefox, VSCode.. But in case all another known me programs they just show File Menu when pressed single Alt and don't process next button like menu item. Another difference in behavior of Opera is very sensitive response to pressing Alt. Just need Alt key down and no mater was pressed another key in same time with Alt you will get menu opening in 90%.
Pressing and holding the Alt key will universally bring up a "File, Edit..." style menu in all windows. However normally it only appears in the top bar whereas Opera is doing a giant drop down menu. For example here is how Firefox behaves when you press and hold the Alt key:
There have been many complaints over the years about Opera's Alt key drop down menu:
So far the Opera developers have ignored the pleas of these users.