Toggling the overflow CSS property in the Inspector removes it when trying to enable it again.
This turns out to be a bug in CSSMutableStyleDeclaration not returning a value for the overflow property, since overflow split into overflow-x and overflow-y and became a shorthand.
I have a simple fix.
Created attachment 22941 [details] Proposed patch
Comment on attachment 22941 [details] Proposed patch r=me
Landed in r35892.