By design, the purpose of the RadPane containing a RadSlidingZone is to wrap it and its size should be one and the same as the RadSlidinZone's size. The desired width have to be set to the used SlidingZones.
When a SlidingPane is docked, the parent RadPane is resized in order to render the docked pane inside and when the SlidingPane is undocked, it takes the SlidingZone's size (because they have to be the same). This comes to say, that the size of the RadPane1 from the above code snippet have to be equal to 22px in order to receive any consistent behavior of the control.
I hope this information will be helpful for you.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to the blog feed