Stravant is correct - the reason this feature is not implemented atm is the requirement to change the UI code to work with arbitrary convex polygons everywhere instead of axis-aligned rectangles.
We can’t use scissor rectangles since they don’t provide full clipping support anyway as mentioned above; the only complete solution that does not need convex polygon clipping is stenciling, but it would require us to de-optimize current rendering code that aggressively merges geometry for visible UI elements to avoid CPU overhead.
We may end up implementing correct clipping some day but it would require a carefully optimized approach that only switches to slow convex clipping when absolutely necessary.