How to fix tumbler: random image thumbnail sizes in Thunar

Solution TLDR;

If you find you have a [PixbufThumbnailer] plugin for tumbler, then you can disable [JPEGThumbnailer].

Apparent reason for the jpg thumbnail size-deviations:

These two thumbnailer plugins seem to be redundant, and even seem to compete with each other when generating thumbnails.

The hint is found in this documentation:

JPEG-thumbnailer

  Is that plugin still needed ? Pixbuf-thumbnailer as well supports jpeg. Is this plugin faster ?

Pixbuf-thumbnailer

  Supports [...] ani, bmp, GdkPixdata, gif, icns, ico, jpeg, png, pnm, qtif, svg, tga, tiff, wmf, xbm, xpm

How to fix:

Edit /etc/xdg/tumbler/tumbler.rc, and find the block:

# Jpeg thumbnailer (from exif data if possible)
[JPEGThumbnailer]
Disabled=false
Priority=3
Locations=
MaxFileSize=209715200

and set the value as follows: Disabled=true.

To let the change take effect, stop the tumblerd process in System Monitor (it will automatically restart on demand), or log out and log back in, or reboot.

Afterwards, new jpeg thumbnails will get generated

• in the right size,
• with the correct portrait vs landscape orientation,

albeit noticeably slower than before.

(Afterwards, only a subset of .pdf thumbnails will suffer from the smaller-than-configured size; I haven't found the fix for that yet.)

To fix all previously generated .jpg thumbnails as well, it's necessary to trigger the re-generating of all thumbnails. This can be achieved by emptying the thumbnail cache:

rm ~/.cache/thumbnails/ -rf

Afterwards, be patient: every image-containing directory you visit thereafter, will start re-generating the thumbnails within; not fun, but worth it.