Size policy of both panes plus the minimum width enforced for the tree pane are responsible of the issue Set horizontal policies to Ignored, and use QSplitter.setSizes() to set a smart default. After this change, it is possible to move the splitter smoothly.
MusicBrainz Picard
MusicBrainz Picard is a cross-platform (Linux/Mac OS X/Windows) application written in Python and is the official MusicBrainz tagger.
Picard supports the majority of audio file formats, is capable of using audio fingerprints (AcoustIDs), performing CD lookups and disc ID submissions, and it has excellent Unicode support. Additionally, there are several plugins available that extend Picard's features.
When tagging files, Picard uses an album-oriented approach. This approach allows it to utilize the MusicBrainz data as effectively as possible and correctly tag your music. For more information, see the illustrated quick start guide to tagging.
Picard is named after Captain Jean-Luc Picard from the TV series Star Trek: The Next Generation.
Binary downloads are available here.
Support and issue reporting
Please report all bugs and feature requests in the MusicBrainz issue tracker. If you need support in using Picard please read the documentation first and have a look at the MusicBrainz community forums.