Thanks for your message.
When we test your code in WPF (because you are using the DockPanel, so I guess you are comparing to WPF) we get the exact same result as in CSHTML5.
Therefore, could you please elaborate on what's the problem and what the expected result is?
Here is a screenshot of the CSHTML5 version (compiled with v1.2 beta 11):
And here is a screenshot of the WPF version:
The two versions appear to be almost identical.
Here is the attached WPF project if you would like to test: