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There is a property controlling the spacing between TabItems called ItemsOverlapFactor. This property is used in our theming for specifying the spacing. ItemsOverlapFactor is a property of TabLayoutPanel - the panel which is responsible for the TabItem layout in RadTabStrip.
To access TabLayoutPanel, you can use the code below:
If you use the property above, the issue you described should disappear. Please write me back if you need additional information.
the Telerik team