Open documents in an application on a different X display

When running multihead with separate servers for each screen (Zaphod mode, not Xinerama), there comes occasional need to launch applications on a display other than the one currently in use.  This comes up in various tasks:

• Launch an app on screen A from a panel launcher on screen B
• Place application windows on various screens with a session recovery script
• Open a URL in a web browser from an application on a different screen

It's the third task which is the solution to a problem I encountered when setting up Thunderbird.  Unless Thunderbird is on the same X display as the browser, It will fail with the error

YOU BROKE IT, MILLIE!

I mention Thunderbird, but this could be Evolution or some other app. The target application doesn't have to be a web browser either.  In fact, if your browser and mail client are on different screens, the browser won't be able to open mailto links in the mail client either.  In these cases, the core of the solution is to temporarily change the $DISPLAY environment variable.

#!/bin/bash
# allows thunderbird to pass URLs to a browser on a different $DISPLAY

cdisp=$DISPLAY
DISPLAY=:0
palemoon $1
DISPLAY=$cdisp

Change the executable name and display numbers to suit. 

For Thunderbird specifically, enabling the use of this script requires changing the attachment handler settings.  Since there is no way to add new entries in the Attachments section of the Preferences dialog (on TB 24), open about:config and set network.protocol-handler.warn-external.http and network.protocol-handler.warn-external.https to TRUE. This forces a user dialog when a link of either type is clicked. Select the new script in both cases. Once successful, the dialog shouldn't pop up again. The resultant settings generated by these actions are visible/editable in the Attachments section of the Preferences dialog. 

It's worth noting that applying these methods to Firefox will open the URL in the existing Firefox session. If you expect to open a new Firefox window on another screen, you're pretty much shit out of luck as far as i know. There are certainly some dodgy workarounds, though.