bugfix: intersected geom should be initiated with a fresh copy of self

frontend/ui/geometry.lua

@@ -117,7 +117,8 @@ returns a rectangle for the part that we and a given rectangle share
 TODO: what happens if there is no rectangle shared? currently behaviour is undefined.
 ]]--
 function Geom:intersect(rect_b)
-	local intersected = Geom:new(self)
+	-- make a copy of self
+	local intersected = self:copy()
 	if self.x < rect_b.x then
 		intersected.x = rect_b.x
 	end
@@ -259,6 +260,11 @@ function Geom:offsetWithin(rect_b, dx, dy)
 	end
 end
 
+function Geom:shrinkInside(rect_b, dx, dy)
+	self:offsetBy(dx, dy)
+	return self:intersect(rect_b)
+end
+
 --[[
 return the Euclidean distance between two geoms
 ]]--