IMPORTANT: This package is not maintained by nor endorsed by any of Geany's developers. It is maintained solely by the Flathub community. Any issues you find in using this package should be filed in the issue tracker here first.
This package currently ships a patch for a change that will officially land in version 2.1, which enables the use of "native dialogs" for file operations. This means the file chooser portal can be used, which will bring up the platform-native file chooser. This package also turns this functionality on by default, in contrast to upstream defaults. Geany's custom GTK3 file chooser does allow for opening files with a different encoding, so if you need that functionality, the relevant setting is at Edit -> Preferences -> General -> Miscellaneous -> Use platform-native file dialogs.