Hello Don Podluzne,
Thank you for writing.
RadToolStrip does not support the described functionality out of the box. However, you could implement it through the SizeChanged event in your own. Please, consider the following code:
sender, EventArgs e)
Size(radToolStripElement1.Size.Width - 300,
Hope this helps. Let me know if you have any other questions.
the Telerik team
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