Simplify code computing max size a pane can be expanded to, in order to support future where there are multiple panes with the same region (ex: two top panes).    As before, a problem remains with the slider disappearing when the center pane has a scrollbar and is shrunk less than about 50px tall, since the browser demands that much space just to draw the scrollbar.
Simplify code computing max size a pane can be expanded to, in order to support future where there are multiple panes with the same region (ex: two top panes).    As before, a problem remains with the slider disappearing when the center pane has a scrollbar and is shrunk less than about 50px tall, since the browser demands that much space just to draw the scrollbar.

Refs #11030 !strict.

git-svn-id: 674a2d6b1e32e53f607eb824547723f32280967e@23379 560b804f-0ae3-0310-86f3-f6aa0a117693

redaktor/deliteful