First, let me explain how the collapse functionality works. When you collapse a RadPane, you actually resize it to 0px width/height. Additionally, in order for a RadPane to resize (collapse), it should be able to resize - e.g. in case you have one RadPane in the splitter, you cannot resize/collapse it; in case you have two RadPanes in the splitter, one of which is collapsed, you cannot collapse the second one as well. In order to be able to resize/collapse a RadPane you need to have an adjacent RadPane that you can resize with the delta that you what to resize the first RadPane (in case you collapse, the delta is the width/height of the first RadPane). That is why, in case you have the following:
- expanded second pane
- collapsed first pane
and you what to expand the first pane and collapse the second on the server, the following happens:
- second pane is initialized and tries to collapse (left border moves). However, the other pane - first pane, is collapsed and thus the second pane fails to collapse.
- first pane is initialized and tries to expand(right border moves). However, the second pane is collapsed and thus it fails to expand.
Thus in your case, you have to control the order in which the RadPanes collapse and expand and additionally, the direction in which they collapse/expand. For the time being, you can specify those only on the client. That is why, I modified your page so that it achieves the desired behavior on the client and collapses/expands the panes in the correct order and with correct arguments - please find a modified version of your project attached.
All the best,
the Telerik team