DISPLAY env variable does not include the Screen number

[MWASoftware]

This is a problem I found after upgrading from Mint 17.3 to Mint 18 (both Sarah and Serena) and seems to prevent normal operation of a dual screen X server. It applies to Mate 1.16.1.

I have a three monitor/two video card setup using Nvidia cards and the Nvidia 375 driver. "Screen 0" is configured with two monitors connected to the same video card, while "Screen 1" is a single monitor connected to the other video card.

After upgrade all monitors are working and appear to be working correctly. Screen 0/Monitor 1 has both a top and bottom panel with an active menu as does Screen 1. Screen 0/Monitor 2 just displays the desktop background. So far, all the same as with Mint 17.3. The screen 0 menus and desktop icons work normally. However, the Screen 1 menu while appearing to work normally: the programs it runs appear not on Screen 1, but on Screen 0! The only way that I can get a program to appear on Screen 1 from the menu system is to add a "Custom Application Launcher" to a panel and select the program I want to run.

Digging further from a mate-terminal session run on Screen 0, I find that the DISPLAY environment variable is set to ":0" instead of the expected ":0.0". If I enter a command such as:

DISPLAY=:0.1 mate-terminal&

the terminal does appear correctly on screen 1 (albeit with the warning message "(mate-terminal:6455): Gdk-CRITICAL **: gdk_display_get_screen: assertion 'screen_num == 0' failed" in the original terminal session.

I can now work around the problem by setting up application launchers on the Screen 1 top panel using command lines that set the DISPLAY variable to :0.1. For example:

sh -c "DISPLAY=:0.1 thunderbird %u"

My conclusion is that the problem is somewhere in mate-session. It is correctly detecting the dual screen setup and creating a dual screen desktop environment, but is not adding the screen number to the DISPLAY environment variable. The result is that X clients always connect to Screen 0 regardless of which menu they are launched from

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[sc0w]

gtk version before (it works) and after (it doesn't work) ?