Previously, when scrolling into an expanded diff fragment from below,

the diff collapse button would disappear off the screen if the diff

fragment took up the entire height of the browser. This was due to the

outerHeight method on the button being treated as a value and not a

function.
Fix an erroneous call to _isMouseOverContainer which should be

_isMouseOverTargetContainer. This fixes a bug where the controls

would not hide sometimes after an expand or collapse finished and the

mouse wasn't over the expanded container.

Tested scrolling upwards and downwards into an expanded diff fragment

in Firefox, Safari, and Chrome on Mac OS X. It worked as expected.