Post | LocalSend Flatpak Hangs on Startup

Symptoms

LocalSend flatpak (org.localsend.localsend_app 1.18.2) updated together with the org.freedesktop.Platform runtime (26.08), and stopped launching afterwards.
Clicking the icon does nothing visible. The localsend process runs but:
main thread spins at ~100% CPU forever
no window ever appears
zero output on stdout/stderr (silent hang, no crash)
Rendering threads (llvmpipe) idle — the busy loop is in app logic, not rendering. LIBGL_ALWAYS_SOFTWARE=1 made no difference (GPU/driver ruled out).
No settings.json is ever written — it hangs during early init.
Port 53317 is free, network interfaces are normal (both ruled out).

Diagnosis

The culprit is the flatpak runtime update, not LocalSend itself:

# hangs on new runtime 26.08
flatpak run org.localsend.localsend_app

# works on old runtime 25.08
flatpak run --runtime-version=25.08 org.localsend.localsend_app

Known upstream issue — the Sep 2026 runtime updates (org.freedesktop.Platform / org.gnome.Platform) broke the LocalSend flatpak (Flutter/GTK engine conflict). Multiple users affected; deb/AppImage builds are NOT affected.

https://github.com/localsend/localsend/issues/3483 — Flatpak freezes on startup after OS/runtime update
Related (similar hang, different cause): https://github.com/NixOS/nixpkgs/issues/348345

Workaround (permanent, until upstream fix)

Point the app back at the old runtime — edit ~/.local/share/flatpak/app/org.localsend.localsend_app/current/active/metadata:

runtime=org.freedesktop.Platform/x86_64/25.08
sdk=org.freedesktop.Sdk/x86_64/25.08

Pin the app and the old runtime so updates/uninstalls don't revert it:

flatpak pin org.localsend.localsend_app
flatpak pin org.freedesktop.Platform//25.08
flatpak pin org.freedesktop.Platform.GL.default//25.08

Check pins: flatpak pin (no args lists them).

Note: pinning also blocks LocalSend app updates. The 25.08 runtime must stay installed (pinning protects it from flatpak uninstall --unused).

Restoring Normal Updates

Once upstream fixes the issue (watch #3483):

flatpak unpin org.localsend.localsend_app \
  org.freedesktop.Platform//25.08 \
  org.freedesktop.Platform.GL.default//25.08
flatpak update   # app update rewrites metadata back to the new runtime

Alternative

Use the tarball/deb/AppImage release instead of flatpak while the bug is open — they are not affected.