I think this request is really a bug:
http://taskcoach.uservoice.com/forums/26465-desktop-version-windows-linux-mac-of-task-coach/suggestions/1378193-toggle-category-enables-multiple-change
It is basically never going to be the case that users want to swap unmarked with marked for a category. They want all bulk items to have or not have a given category. The standard behavior (for example in marking bold text in a word processor) is to first mark any unmarked of the selection, and then once everything has the marking, the next action removes from all.
I realize now, especially considering this request: http://taskcoach.uservoice.com/forums/26465-desktop-version-windows-linux-mac-of-task-coach/suggestions/2159641-activation-of-categories-tab-for-multiple-tasks
that there is no functionality at all for multiple task category editing. So the fact that toggle category can be chosen by right-click or action menu when multiple tasks are selected is an accident, and basically a bug.
The tiny lousy solution is to remove that option when multiple tasks are selected.
Better, of course, is to actually implement category toggling for multiple tasks, but that is more of a feature request I suppose…
I don't agree that being able to apply toggle category to multiple tasks at the same time is a bug. That being said, I do think that it's unlikely that a user selects two tasks, e.g. task 1 in category A and task 2 not, and then wants to have task 2 in category A and task 1 not.
So how about this:
Exactly!
To simplify: