bug#24084: 25.1; GTK3 mode-line drop down menu partially off-screen

[Sebastian Schueppel]

This is related to a multi monitor setup: I have 3 monitors set from left to 
right. The middle monitor has a vertical orientation therefore the pixels 
available on that one is swapped. The monitors have the following resolutions: 
(1680x1050)(1050x1680)(1680x1050)
If I do the following on one of the horizontal/normal oriented monitors the 
mode-line drop down menu disappears partially off-screen.

starting from 'emacs -Q'

maximize the frame on one of the monitors with normal orientation (1680x1050) 
and click in the mode-line on the line number or column number. The 
popup/dropdown menu will partially disappear at the bottom of the monitor. 

--
My debug/quickfix so far: 

The popup menus like this are defined in emacs/src/xmenu.c 
Since I'm using GTK3 the 'menu_position_func' function is always used to define 
the position of the popup menu. Which is certainly the correct thing to do, 
otherwise GTK3 does sometimes some funky stuff. Now in my 3 monitor set-up, the 
function within 'menu_position_func' called 'x_display_pixel_height' always 
thinks the height of my screen is 1680 and therefore thinks there is room to 
display the menu below. It doesn't take into account that one monitor is 
vertical oriented and has a height of 1680, but the other 2 monitor are set up 
normal with a height of 1080. 

I have used the following quickfix using GTK3 build in functions using monitors 
rather than the whole screen information. Adding the following to 
'menu_position_func' to get the geometry of the monitor the frame is on. I 
understand that this might not be generic enough, but it works for me. 

<code>
#include <gdk/gdk.h>

GdkRectangle workarea;
// get the monitor the current frame is on
gint widgetMonitor = 
gdk_screen_get_monitor_at_point(gtk_widget_get_screen(FRAME_GTK_OUTER_WIDGET 
(data->f)),*x,*y);
gdk_screen_get_monitor_workarea(gtk_widget_get_screen(FRAME_GTK_OUTER_WIDGET 
(data->f)),widgetMonitor,&workarea);
// now instead of using disp_width and disp_height we use a combination of 
monitor absolut position and monitor working area.
// monitor absolute position being  workarea.x workarea.y and monitor size 
being workarea.height and workarea.width
</code>

I'm omitting the rest of the code here where those values are used as it should 
be obvious. Hope this will help to get a more generic solution out there. 



In GNU Emacs 25.1.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2016-07-26 built on mymachine
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Windowing system distributor 'The X.Org Foundation', version 11.0.11501000
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