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?
Best Answer
GNOME Connection Manager is intercepting the Alt key combinations for its own menu and refuses to pass it on to the shell it is hosting (where your MC instance is running). This is not configurable.
This is a bug in GNOME Connection Manager, or to be precise in the GUI library it uses (Gtk), so the only good solution is to go to their site and report the bug.
The other approach is filing a bug with GTK about "Alt being hardcoded" but you will need a lot of charisma and endurance to convince them to make Alt configurable. All alternatives will be workarounds, of which the ESC-key trick is the most elegant.