I've started just playing with the subscreen to see what bugs I can stir up in 536.
Here's a start.
When you choose to make a new subscreen and choose blank to start, and active, shouldn't the window (the black section) be the size of the other active subscreens and not that of a passive subscreen?
Clicking on "switch active/passive" makes it the large window, but I would think choosing active to begin with would have set that.
And clicking on the canvas area (in the black) and then choosing properties results in this;
deleted pics.