bug#52809: 28.0.90; X11 modeline context menu grows offscreen unreadable on smaller screen of two screen display

[Van Ly]

Hello,

On the modeline, on the "Major mode" keyword, for example
"Fundamental", mouse button 1 and 3 cause a context menu to show
overlapping the modeline.

On a two screen display of uneven size, on the smaller of those two
screens, when the X11 frame for emacs is maximised fullscreen after
using the F11 key for toggle-frame-fullscreen, that context menu
grows offscreen and unreadable.

Similary, on the smaller screen, an X11 frame for emacs with the 
bottom of the frame put near to the bottom of the smaller screen, 
that context menu grows offscreen and unreadable.

# steps to reproduce

## maximised X11 frame for emacs on smaller of two screens

1. start emacs by "emacs -Q"
2. move the emacs X11 frame to the smaller screen of a two screen 
GNU/Linux LXDE
3. press F11 for toggle-frame-fullscreen
4. press button mouse-1 or mouse-3 on the "Major mode" keyword for 
context menu

## not maximised X11 frame for emacs on smaller of two screens

1. start emacs by "emacs -Q"
2. move the emacs X11 frame to the smaller screen of a two screen 
GNU/Linux LXDE
3. move the bottom of emacs X11 frame to bottom of the screen
4. press button mouse-1 or mouse-3 on the "Major mode" keyword for 
context menu

# observed behavior
The context menu grows offscreen and unreadable.

# expected behavior
The context menu "floats" as it grows, showing all the rows on the 
context menu.


--
vl