Moved the Flatpak display troubleshooting content from readme.md into a standalone troubleshooting.md document, as requested.

Replaced the large inline README section with a single link to the new troubleshooting file to keep the README concise.

readme.md

@@ -36,6 +36,9 @@ See [IRCHelp.org](http://irchelp.org) for information about IRC in general.
 For more information on ZoiteChat please read our [documentation](https://docs.zoitechat.zoite.net/):
 - [Downloads](https://zoitechat.zoite.net/download)
 
+- [Troubleshooting](troubleshooting.md)
+
+
 ---
 
 <sub>

troubleshooting.md

@@ -0,0 +1,41 @@
+# Troubleshooting
+
+## Flatpak
+
+If `flatpak run net.zoite.Zoitechat` only prints `Gtk-WARNING **: cannot open display`,
+collect extra diagnostics with:
+
+```bash
+flatpak run --devel --command=sh net.zoite.Zoitechat
+```
+
+Then inside that shell:
+
+```bash
+echo "DISPLAY=$DISPLAY WAYLAND_DISPLAY=$WAYLAND_DISPLAY XDG_SESSION_TYPE=$XDG_SESSION_TYPE"
+xdpyinfo >/tmp/xdpyinfo.log 2>&1 || true
+env G_MESSAGES_DEBUG=all zoitechat 2>&1 | tee /tmp/zoitechat-debug.log
+```
+
+To inspect sandbox permissions from the host:
+
+```bash
+flatpak info --show-permissions net.zoite.Zoitechat
+flatpak override --user --show net.zoite.Zoitechat
+```
+
+If needed, try running with direct access to your active display stack:
+
+```bash
+# X11 sessions
+flatpak override --user --socket=x11 net.zoite.Zoitechat
+
+# Wayland sessions
+flatpak override --user --socket=wayland net.zoite.Zoitechat
+```
+
+You can reset overrides after testing:
+
+```bash
+flatpak override --user --reset net.zoite.Zoitechat
+```