scrollmorph doesn't update scrollbars when morph is attached inside it

[davidedc]

steps:

• open scrollframe
• open a morph so it only partially overlaps
• attach the morph to the frame inside the scrollframe

like so:

obtained:

• the part of the morph outside the scrollframe is not cleared
• the scrollframe doesn't show scrollbars

expected:

• the part of the morph outside the scrollframe should be cleared
• the scrollframe should update its scrollbars

(this is fixed in Zombie Kernel but can't reference exact commit, bisection might be necessary)