Complex sentence: A new class bordered-button which is both a graphical object and uses the border-mixin would compute its width by calling all applicable width methods-via the + method combination all return values are added and create the combined width of the object.
Split into simpler sentences: A new class bordered-button (which is both a graphical object and uses the border-mixin) would compute its width by calling all applicable width methods-via the + method combination. All return values are added and create the combined width of the object.