Feature: Allow second pane to keep size when owner resized

[JanBliznicenko]

What I need is to have right/bottom-side toolbar with fixed size considering outside source of resize (like resizing window), but still allow manual resizing by using the splitter. It would also allow to have containers like this: https://youtu.be/pO4J6JflZeg?t=10

Currently, if two panes with a separator are used (for example by SpPanedLayout) and I set fixed position of separator (ie. not percentage) and then their owner is resized (for example by resizing the window or some other outer pane), the first (left/top) pane keeps its size and the second (right/bottom) one is being resized to compensate for the new owner size.

I would like to be able to set position not from the left/top (size of the left/top pane), but position from the right/bottom (size of the right/bottom pane) and once I do, when owner/window is resized, the left/top pane would be resized, not the right/bottom one - it would keep its size like a proper toolbar, yet still allow to be resized using separator.