In vanilla, there are a few useful mouse actions & gestures that are usable in most inventories. The following example video covers, in order:

• right-click single spreading
• left-click stacks spreading (with even distribution)
• double-click "collect all of held type"

WSlots do not follow the same behavior, as shown in the following video (same order):

yes, this is something i tried to implement but ended up too cursed. i plan on fixing it.