Most Adobe Flash applications that have a fullscreen mode will exit back to their original in-webpage state when the fullscreen window looses the focus (or is not the active window). This is fine on a single monitor setup but a real pain on a dual or multi-monitor system. Often the user would like to watch a Flash based movie on one monitor (e.g., Youtube) and continue working (e.g., browse the web) on another.
In my case, I certainly want to be able to put the Flash application into fullscreen mode then interact with other applications on another monitor while a Flash video plays fullscreen. Fortunately I found a solution for my Ubuntu Linux system.
The approach that is working well is to criple the Flash application's ability to determine the active window. If the application can't detect an active window change then it won't leave fullscreen mode.
This is done by a single byte edit of the Flash program file.
Using a binary or hex file editor (like "hexedit"), open libflashplayer.so. On my Ubuntu system, the command is:
sudo hexedit /usr/lib/flashplugin-installer/libflashplayer.so
Next, switch to ASCII or text edit mode with the keystroke tab, then search for the character string "_NET_ACTIVE_WINDOW". In hexedit this is done with the keystroke sequence, ctrl-s, NET_ACTIVE_WINDOW.
Now, change any one character in the string to another character. In my case I changed the string to "__ET_ACTIVE_WINDOW".
Save the file and exit the editor. In hexedit this is ctrl-x.
The result should be that fullscreen Flash applications do not exit the fullscreen mode when they loose the focus. Instead you'll need to click on the fullscreen window and use the application's exit fullscreen key, which is typically the esc key.
Alternatively, you can accomplish this same change in one command with:
sudo sed -i 's/_NET_ACTIVE_WINDOW/__ET_ACTIVE_WINDOW/g' /usr/lib/flashplugin-installer/libflashplayer.so
Note, I use Firefox for web browsing and therefore also for running the Flash plugin. I believe the approach will work with other browsers but I have not tested.
Enjoy your movies!