Tiny unclickable section at top of menubar

[protist]

I'm using this kwin script to hide borders when a window is maximised. If I maximise Dolphin, then I can push my mouse cursor to the top of the screen, then click to activate the menus.

However, if I run Gnumeric with Breeze-gtk active, there appears to be a tiny "unclickable" region at the top of the menubar, which is about 2 pixels wide. This means that the mouse cursor cannot be on the screen edge to activate menubars in GTK applications. This is slightly more fiddly, and inconsistent with native Plasma/Breeze menubars.

Plasma 5.3.2
Arch Linux
gtk3 3.16.6
gnumeric 1.12.2