setFollowOffset method
Nudges the open menu to track a moving anchor: offset (screen px) is added
to the menu's captured trigger position every frame until reset. A no-op
while closed; cleared on the next open. Used by external gesture owners
that move the anchor AFTER opening — e.g. a canvas tile trailing under a
rubberband, where the menu should stay glued to the tile.
Implementation
void setFollowOffset(Offset offset) => _state?.setFollowOffset(offset);