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outliner uses ctrl click to select multiple items
viewport uses shift click
It would be better if both used ctrl click for consistency.
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Another slight inconsistency is box deselect. At the moment you can box select by just holding the left mouse and dragging, but to box deselect you have to enter the box select modal by pressing B and then use middle mouse instead of left mouse. Would it be better to just alt drag to box deselect rather than the current ctrl + drag? This would be more intuitive as alt is used to perform the inverse operation in most other areas of blender.