It would probably mean setting aside a block of the GPU threads and Vram for GUI which in turn would take them away from the render process. I suspect that some people would not be keen if they are rendering “at the limit” of their hardware possibilities, it could be preferable to leave the GUI unresponsive (“wait I am busy”) than to run out of resources completely.
I think it would be more a “suggestion” than a bug, but I expect it would be complex and there would be pros and cons.