Fixing problems with tests running with XUnit StaFact

[eriove]

I ran into a problem where my tests were failing when updating to version 4.30.0 or later. I got an exception in DockingManager.OnContextMenuPropertyChanged(). It was introduced as part of
#170 to solve a problem with the context menu.

Here is a link to the specific change (you will have to expand the diff of DockingManager to see the row).

My problem is that I'm running tests with the DockingManager in XUnit with StaFact. StaFact makes sure that the tests run on on STA thread. But it doesn't guarantee that all tests within a class runs on the same STA thread. Hence d will have the test thread's dispatcher, but for some reason the contextMenu will have a dispatcher from a thread from a previous test (from within the same test class).

This is a hack (and I'm not sure if it should be merged) but the only way I can think of resolving this right now. @LyonJack (or someone else) do you have any ideas why the context menu end up on another thread? Is it somehow reused for multiple docking managers? Dependency properties are static so I can see how that could happen.

[eriove]

I've finally had time to dig deeper into this problem and updated the pull request.

The problem is that the styles are statically cached and will be re-used between tests. If different tests run on different threads they will crash as described above. I've created a minimal test that reproduces the problem, it is available here.

By changing the context menus from static to dynamic resources the tests pass, but there is some extra resource allocation. @Dirkster99, do you think that that would an acceptable trade off?

[eriove]

Thanks for merging!

[Dirkster99]

Thanx for contributing :-)