Selecting a menu item using the mouse at the top edge of the screen, then moving between top-level menu items while holding my left mouse button down doesn't select those menu items. To do so, it is necessary to move one pixel down before moving left or right. This makes it significantly more difficult to move between menus.

STEPS TO REPRODUCE
1. Place Global Menu at the top edge of the screen.
2. Move mouse against the top edge of the screen and select the left-most top-level menu item (e.g. the "File" menu on many programs). Click and hold the left mouse button.
3. Now, still holding the left mouse button down, and keeping the mouse pointer against the edge of the screen, move the mouse to the right until it is over the next top-level menu item.

OBSERVED RESULT
The left-most menu item stays selected.

EXPECTED RESULT
The next menu item should be selected.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Debian GNU/Linux 10
(available in About System)
KDE Plasma Version: 5.14.5
KDE Frameworks Version: 5.54.0
Qt Version: 5.11.3

ADDITIONAL INFORMATION
The expected result seems not to occur because there is a one-pixel border around the Global Menu. This border does not effect the initial selection of a menu item, but does affect subsequent selections, if the mouse button is held down. 

The workaround is to move the mouse down by one or more pixels before moving to the left or right.