Before the RC0 we could successfully change the ZIndex of popups, while in the latest version this is not always possible (changing the ZIndex of the Popup child does not work either).
There is a very strange bug that appears from time to time when the Popup is closed also.
We found workarounds for most of the "breaking changes" but it seems this one introduced more problems than we expected.
When the popup behavior returns to normal (as it was before and as is currently in WPF), the TopMost problem will disappear. Until then we will do our best fix that.
Thanks for bringing up this issue!
All the best,
the Telerik team
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