How to Hack gtk3-nocsd Into Flatpak Applications

WARNING: Use at your own risk. This is NOT a supported configuration and you’ll want to turn this off and re-test before reporting any bugs. (i.e. Report bugs gtk3-nocsd fixes, not ones it causes.)

OK, so you’ve got a desktop that’s using gtk3-nocsd and a GTK app that renders fine outside Flatpak, but does something unacceptably CSD-ish inside Flatpak. (For me, among other things, it’s non-GNOME GTK-using applications like Deluge displaying big black borders on things like context menus when KWin’s compositing is disabled.)

TIP: If it’s only the shadows on the context menus you don’t like, then it’s easier and more reliable to toss decoration, decoration:backdrop { box-shadow: none; } into your ~/.config/gtk-3.0/gtk.css.

The Flatpak developers don’t appear to see this as an issue, and not all applications have a good Qt-based counterpart, so what’s a stubborn “It’s my ****ing computer” person supposed to do?

…what about taking advantage of how Flatpak applications were already probably printing this to the terminal every time you launched them?

ERROR: ld.so: object 'libgtk3-nocsd.so.0' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.

Here’s how you shoe-horn your gtk3-nocsd into Flatpak without rebuilding the Flatpak Runtime or the application package:

1. Create a folder that Flatpak isn’t going to overlay, like /gtk3-nocsd and make sure it’s got the same 0755 root:root permissions as anything else nobody but root should be able to muck in.
2. sudo cp -p /usr/lib/x86_64-linux-gnu/libgtk3-nocsd.so.0 /gtk3-nocsd (substitute whatever path your host distro uses for the original, non-symlinked library file and note the -p to preserve the permissions and ownership. You need the sticky bit to pass certain safety checks.)
3. Go into Flatseal or use flatpak override and grant your application filesystem access to /gtk3-nocsd:ro, then add an environment variable override to LD_PRELOAD=/gtk3-nocsd/libgtk3-nocsd.so.0

Congratulations. For better or for worse, you should now have your Flatpak’d application loading gtk3-nocsd like an un-Flatpak’d application. It seems to have solved the “big black borders on context menus with compositing disabled” issue in Deluge-GTK for me… but I make no guarantees that it’ll work for you.

You can check if it’s getting loaded by:

1. flatpak enter <app ID> sh
2. ps ax to get the PID of the process (probably 2 in my experience)
3. grep nocsd /proc/<PID>/maps

In theory, you could do this in Flatseal’s “All Applications” tab, but why tempt fate. Do as I did. Apply it selectively to just the applications that exhibit problems.

I doubt the Flatpak devs will ever enable ld.so‘s secure execution mode, since it disables LD_LIBRARY_PATH, which is used in some launcher scripts to make some applications find their libraries under /app rather than /usr/lib (I had to use it to get a custom build of SDL_mixer to find libmodplug at runtime even though it found it without issue at compile time) but, if so, the next step would be to try injecting gtk3-nocsd.so.0 into /var/lib/flatpak/runtime/org.freedesktop.Platform/x86_64/<VERSION>/active/files/lib/ so it’ll show up in the list of standard paths ld.so‘s secure mode is whitelisted to.

How to Hack gtk3-nocsd Into Flatpak Applications by Stephan Sokolow is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.