Frame shadow of gtk3 application is not transparent for mouse clicks

This affects gtk3 indicator in Xubuntu (xfwm or mutter as wm). Usually the black borders in the attached screenshot are transparent shadows, but it represents exactly the area which is sometimes not clickable.

To reproduce:
* click a gtk3 indicator
* hover over the expanded indicator menu
* try to click the indicator or any neighbors
=> only clicks in the upper ~1/3 of the button show a reaction, the lower 2/3 do nothing.

The same thing happens with some gtk3 window shadows, e.g. the gnome calculator: Underlying windows cannot be brought into focus near the window border in the shadow area.

This bug has been reported on the Ubuntu Package testing tracker.

A list of all reports related to this bug can be found here:
http://packages.qa.ubuntu.com/qatracker/reports/bugs/1521586

So, its not the shadow actually, which would have been weird in the first place. But some apps (I would say gtk3, csd; eg. gnome-calculator, gnome-software) show a different behavior concerning window border grabbing (for resizing). Windows, which can be resized, change the cursor the discussed area next to the window border and can be resized. This behavior is probably also applied to windows, which can not be resized, like gnome-calculator or the indicator menus. Thus leading to the several pixel wide region near window borders, where mouse clicks are not interpreted as, well, clicks.

If the border grabbing margin is intentional (although it leads to an inconsistent behavior between different apps), then it should be disabled for windows, which are not resizeable.

I should mention, that I get this issue only with enabled compositing in the wm. In my case with xfwm or mutter (cf. attached screenshot).

Can confirm this part

>The same thing happens with some gtk3 window shadows, e.g. the gnome calculator: Underlying
> windows cannot be brought into focus near the window border in the shadow area.

Ubuntu 18.04 gnome shell, multiple monitors, nvidia.

I first noticed as I had a window full screen on monitor 1, and it prevents the use of the "close,minimize,maximise" controls on the titlebar of neighbouring window on another monitor, unless I focus on it, by clicking outside the shadowed region.

I'm not 100% on who is at fault, but shadows of UI elements should be transparent to clicks, IMO.

Status changed to 'Confirmed' because the bug affects multiple users.