When I use GNOME Shell, and GNOME Connection Manager program, and try Alt+S in Midnight Commander (all inside GNOME Connection Manager), I get a window context menu opened, instead of the search action shortcut in MC. This is just an example: it's an issue in other programs too.
I cannot fix this with keyboard shortcuts settings as described in this Ask Ubuntu answer. There is no such menu item as Keyboard Shortcuts.
Also using the trick with the Escape key is not good, as it's a workaround rather than a real solution.
How can I simply disable this particular Alt key behaviour?