PopupBox and DialogHost appearing on top of other windows

Nobody has claimed this yet.

Video of bug: https://gfycat.com/exhaustedthosebengaltiger

I've had lots of users reporting this issue to me. It seems to happen with both PopupBox and DialogHost. I suppose it will also happen with other popup elements.

I was not always able to always reproduce the bugs so I'm not sure what could be cauing it to happen.

The PopupBox bug seems to happen when I reopen my Window from the Tray with **Show()**.

The PopupBox in my app has the **MaterialDesignMultiFloatingActionPopupBox** style set on it and no other special properties.

Steps:
* Open your App
* If you're reproducing the DialogHost then make sure to **Show** it
* Open a Window on top of it
* If you are reproducing the DialogHost then it should be opened on top of the window you opened **(This will not always work)**
* If you are reproducing the PopupBox then hover over it with your mouse **(This will not always work)**

Start by reproducing the reported behavior with PopupBox and DialogHost using the listed steps, including reopening a window with Show() and applying the MaterialDesignMultiFloatingActionPopupBox style. Compare behavior when another window is opened and when the popup is hovered. Done means the reported popup elements no longer appear on top of other windows.

Written by the indexing model from the issue text.