XnView Software

XnViewMP 1.9.8
Windows11 (25H2), display resolution 3840x2160, dpi scale 150%

Browser - Album
its possible to add folders into Albums (if this is intended functionality ),
but 'Show files in subfolders' does not work, which is inconvenient

Suggestion

• add 'Show files in subfolders' functionality
  to see content of all folders withing single album

Related

• issues
  1.9.8 - browser - album - issues with adding objects

• suggestions
  Browser - Album - add flattened view (current post)
  Browser - Album - add UI controls

FYI, you can get this effect using Smart Albums by utilizing the Pathname or Folder Name conditions, albeit without the ability to add individual arbitrary images with drag and drop.