This is a rough patch to allow cursors past the end of the line when the "new block mode" action is checked. It doesn't improve that much upon the "it's fully broken" current state, but at least restore the capability to select past the end of the line and create multi-row vertical cursors using the keyboard.
This is a "code dump" more than a "I will be responsive forever about this code review". I said I would fix this and this patch does it. I hope it helps give some energy back into this branch and I hope it gets merged in the future.