Okay, time for an actually useful feature: interactive mouse resizing (yes, finally). This was, as it goes, quite fiddly to implement, especially since niri has to negotiate with the window during the process.
I also added a double-resize-click (i.e. trigger a resize twice quickly) gesture to reset the window height or to toggle full width. Suggested by FreeFull on our Matrix and worked out very well! Really starting to feel quite nice with mouse.
(still no transactions yet)