As the problem lies in the size passed to the TimerulerPart of the RadGanttView the only workaround I can suggest is to force a remeasure.
For example cause a remeasure in the parent window:
//For the MainWindow to invalidate its measure:
// Change the PixelLength of the control
// Force measure invalidate
Another approach might be to change the selected item of the RadGanttView to a task outside of the current viewport and then set it back to the initial selected item - this way a scrollIntoView of the panel will be called and the Timeruler part will extend.
Hope this helps.
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