The library-folder picker does open and Steam does create the directory
structure; the library simply never appears in the Storage UI. That rules
out the portal FileChooser for this symptom and points at the
write-and-execute probe Steam runs before recording the folder in
libraryfolders.vdf (ValveSoftware/steam-for-linux#8450). Document how to
reproduce that probe inside Steam's FHS sandbox with steam-run, and keep
library_folder_add as the route that reports a reason.
The Add a Non-Steam Game dialog is a separate symptom and keeps the
portal explanation.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
