fix: controls activating when overlapping chapter is clicked (#373)

scripts/uosc_shared/elements/Button.lua

@@ -24,6 +24,8 @@ end
 
 function Button:on_coordinates() self.font_size = round((self.by - self.ay) * 0.7) end
 function Button:on_mbtn_left_down()
+	-- Don't accept clicks while hidden.
+	if self:get_visibility() <= 0 then return end
 	-- We delay the callback to next tick, otherwise we are risking race
 	-- conditions as we are in the middle of event dispatching.
 	-- For example, handler might add a menu to the end of the element stack, and that