RFR: 8155030: The Menu Mnemonics are always displayed for GTK LAF

[Tejesh R]

On Mon, 29 Apr 2024 09:00:46 GMT, Abhishek Kumar <abhiscxk at openjdk.org> wrote:

> In GTK LAF, the menu mnemonics are always displayed which is different from the native behavior. In native application **(tested with gedit**), the menu mnemonics toggle on press of `ALT` key. Menu mnemonics are hidden initially and then toggles between show/hide on `ALT` press. 
> Proposed fix is to handle the `ALT` key press for GTK LAF and mimic the native behavior. Fix is similar to the `ALT` key processing in  Windows LAF. Automated test case is added to verify the fix and tested in Ubuntu and Oracle linux.
> 
> CI testing is green and link attached in JBS.

The fix applies to single/first menu, its doesn't apply to other Menu items when there are more than one. Since you have taken _gedit_ as reference where one Menu item is present you might have followed according to it, but when you see other native applications like _Libre_, on press of Alt key all the Menu Mnemonics are toggled. It would be better to consider update the fix for all Menus.

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PR Review: https://git.openjdk.org/jdk/pull/18992#pullrequestreview-2042783043