rAudio 20260820: (Minor) issue in thumbnail generation

[klassikmann]

Short explanation:
Parts of the music library containing a folder.jpg but no music files (which are contained in sub-directories) don't get thumbnails an more ("Update thumbnails").

Long explanation:
I'm organizing multi-volume works containing different components (e.g. an opera in multiple acts) in such a way that the parent directory contains the sub-directories containing the different components plus the booklet of the multi-volume work (if available) and the 'folder.jpg' but no music files which are contained in the sub-directories plus 'folder.jpg'.

Let me explain the issue with an example.

Parent directory:

 #  ls -1 Cancioneros\ del\ Siglo\ de\ Oro/
'1. El Cancionero de la Colombina (1451-1506)'
'2. El Cancionero de Palacio (1474-1516)'
'3. El Cancionero de Medinaceli (ca 1533-1595)'
Booklet.pdf
folder.jpg

One of the sub-directories:

'01. Di, Perra Mora.flac'
...
...
...
'17. Ojos Claros Y Serenos.flac'
folder.jpg

After the "Update thumbnails" the sub-directories contain the thumbnails (coverart.jpg, thumb.jpg) as before, but not the parent directory. This behaviour distinguishes the former approach.

[rern]

Thumbnail of directory / folder:

• coverart.jpg (png/gif) - album list and bookmark
• thumb.jpg for directory list

Coverart for Playback and album track list

• names: cover, album, folder, front
• extensions: jpg, png, gif

[klassikmann]

@rern
I described a change regarding thumbnail generation that is incompatible to all previous releases. The change will be visible if modelling operas in the music library:

│   ├── <Composer>
│   │   ├── <Opera>
│   │   │   └── <Conductor>
│   │   │       ├── 1. Act
│   │   │       ├── 2. Act
│   │   │       └── 3. Act

The same 'folder.jpg' is present in directory and the three subs. An "Update thumbnails" at the directory creates thumbnails in the three acts directories but no more in the directory (as before).

Conjecture: The regression is a side effect of fixing the issue #2150.