SplitButton Popup Doesn't Close After Alt+Tab

[Deadpikle]

What steps will reproduce this issue?

1. Open demo app
2. Click dropdown arrow button on the split button in the bottom right (the one that says "Worlds")
3. Alt+Tab a few times
4. Now you can't close the Popup without selecting an item in the Popup's list, even when selecting another control.

On a side note, it'd be good if the popup closed on Alt+Tab as well, probably, since it shows over all windows.

Expected outcome

Expect the popup to close even if the window has lost focus and/or I've come back to it.

Environment

• MahApps.Metro v1.3.0-ALPHA174 (built from commit 1b12418)
• Windows 10
• Visual Studio 2015
• .NET Framework 4.5

Notes

OutsideCapturedElementHandler in SplitButton.cs doesn't seem to be called. That's probably the reason this is broken. :(