Pane test. Pane is not restored after having been hidden

[scaramacai]

Tried pane test in agartest. Steps:

• start from the default situation
• move the separator to the left
  • left side of the pane is now hidden
• move the separator back to the right
  • the left side of the horizontal pane is shown again
  • the vertical pane separator has changed position
    • only the left/bottom is shown

Setting the UNMOVABLE flag to True does not change things.

An analogous behavior can be observed if the roles of the horizontal and of the vertical panes are exchanged.