uncontrolled constant Null safety

A special value that doesn't change cursor by itself, but make a region that blocks other regions behind it from changing the cursor.

When a pointer enters a region with a cursor of uncontrolled, the pointer retains its previous cursor and keeps so until it moves out of the region. Technically, this region absorb the mouse cursor hit test without changing the pointer's cursor.

This is useful in a region that displays a platform view, which let the operating system handle pointer events and change cursors accordingly. To achieve this, the region's cursor must not be any Flutter cursor, since that might overwrite the system request upon pointer entering; the cursor must not be null either, since that allows the widgets behind the region to change cursors.