AvalonDock: Hidden anchorable cannot be shown again

[Dirkster99]

xceedsoftware/wpftoolkit#1457

Consider specific use case:

When invoking user actions in the order:

Hide Screen3 // LayoutAnchorable.Hide()
Close Screen2 // LayoutAnchorable.Clolse()
Show Screen3 // LayoutAnchorable.Show()

the Screen3 does not re-appear.

When invoking only steps 1 & 3 - the Screen3 appears as expected.

Sample project attached: Test-bug.zip

[Dirkster99]

The solution to this problem is quit involved.

The problem appears to be that the Docking framework has a:

• LayoutContent.PreviousContainer property
• ILayoutPreviousContainer -> LayoutAnchorGroup.PreviousContainer

which are uesd to relate items back to where they where before they are being hidden. These properties are necessary because the LayoutRoot clears out empty container and implements a CollectGarbage() method to really remove hidden items from memory and view:
LayoutRoot.CollectGarbage()

But the CollectGarbage() method also removes items that appear to be empty but are referenced via the LayoutContent.PreviousContainer property.

This is why this resolution adds another guarding statement in the LayoutRoot.CollectGarbage() method to not remove a LayoutAnchorGroup if it appears to be empty but is still referenced via the LayoutContent.PreviousContainer property.

To make this work, the code that invokes the LayoutRoot.CollectGarbage() method in LayoutContent.CloseInternal() is extended such that a PreviousContainer property is changed to its Parent property if it points to an element that is likely to be removed next.

Additional testing has shown that the LayoutAnchorable.Show() method has to be extended with a:
Parent = previousContainerAsLayoutGroup; stement in order toggle its property from:

• this.IsHidden == true to
• this.IsHidden == false

All required changes for this solution can be found in a seperate branch:
https://github.com/Dirkster99/AvalonDock/tree/Issue19

[Dirkster99]

Fixed with
#22
#23