Hello !

"No there isn't any facility in v5 for doing a toggling selection window that would result in a mix of both selecting and also unselecting within the same window action. Do you have an example where that would be useful?"

Well, the most immediate example would be ... the regular click (or shift-clicking, depending on which option is enabled) for selection. It already performs such a toggle of selection state, since clicking (or shift clicking) on a selected element deselects it. So it wouldn't be much of a stretch to expect the marquee selection to perform similarly too.

But, this would have been mostly relevant (as a potential solution for marquee-deselect) had the ctrl behavior not been adressed in V5. It wouldn't hurt to have as an option though, as that would mean that people could optionally manipulate selections (by adding and removing elements to/from them) using just one modifier key (shift) instead of two (shift and control). That said my experience is mostly based on 2D drafting using MOI, so without a doubt there are some potential conflicts in 3D that I may not be seing. And shift to add / ctrl to remove is near universal anyways.

Overall if I were to pick only one thing to tweak/add to the system, it definitely would be the option to not have shift-click cause any deselect ever when performed over empty space, as this alone would personally fix my frustration of often losing selections because of missclicks. And while undo is indeed a remedy, it still doesn't make the frustration of losing a selection retroactively disappear :D And slowing down is not an option ...