Old code to prevent multiple overlapping regions at the same place,
wasn't handling the combination of bit flags and the alignment enum in a
single field correctly. Newly introduced flags for the asset shelf
exposed this.
Was asserting on startup for a Pets production file
(`010_0050.anim.blend`).
Steps to reproduce:
* Replace default cube with armature object
* Enter Pose Mode (makes asset shelf available for pose libraries)
* Hide 3D View header
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