LyX is by default set to store temporary files in the path '~/Documents/.tmplyx', which can be changed from the 'Tools->Preferences->Paths' menu. In this path, LyX stores pdfs, if it is not accessible it will use the '/tmp' folder. The path must be accessible from outside the sandbox or it will not be possible to use an external pdf viewer. If you have problems viewing pdfs, try:

• Granting access permissions to the path where you want temporary files to be stored. You can do this with Flatseal or with commands:

  • sudo flatpak override org.lyx.LyX --filesystem=path_to_temporary_directory

• Set that path for temporary files in LyX 'Tools->Preferences->Paths->Temporary directory'

If you want to install lyx-2.3.7-1 from the 'stable' branch, you can do so, although you will need to complete the dependencies by hand:

• Download LyX
  • flatpak install --no-related org.lyx.LyX
• Revert changes to v2.3.7-1
  • sudo flatpak update --commit=5bea7d745bdcd0cbbeda016068379833024f5c20c66e839fead875518752b242 org.lyx.LyX
• Install dependencies:
  • flatpak install org.freedesktop.Sdk.Extension.texlive//23.08
  • flatpak install org.kde.Platform//5.15-23.08 If LyX does not run, you may need to remove the configuration files in the ~/.lyx path inside the container.