[regression] AccordionContainer: temporary layout problem on pane open (IE)

[bill]

On IE, often upon clicking an accordion pane to open it, after the animation finishes it will be in a weird state:

Sometimes the problem is more subtle, that the bottom border and the 1px margin below it aren't visible:

In either case, moving the mouse away from the pane and then back over the pane fixes the problem.

[bill]

(In [22439]) workaround IE layout glitch upon opening pane, fixes #11415