System Information
Operating system: win10 pro - 64bit
Graphics card: GeForce GTX 960

Blender Version
Broken: blender-2.83

Short description of error
Box-selecting vertices in weight paint mode behaves as if the deformation of the armature isn't applied.

Exact steps for others to reproduce the error

• Open:

• Use Box selection

or:

• Start with a model weighted to a armature with a pose != restpose (e.g. take the file I have uploaded)
• Model needs a Mirror modifier applied before the armature modifier (see example) (without modifier it will work as expected without bug)
• Activate X-Ray view mode (without it will work), viewport shading doesn't seem to matter
• Select model and go into weight paint mode
• Activate vertex selection (Face selection will work)
• Choose Select Box mode (Single click selection will work)
• Try to precisely box select one or multiple vertices which are transformed by the armature pose - it will not select them or it will select other vertices depending on your box selection and how far the vertices are transformed from their rest position.
• Now try to precisely box-select them at their rest position - they will be selected