I built up a test demo page, based on your code but unfortunately I was not able to reproduce the problem. I had to remove your user controls declarations and after I ran the page I could not see the problem.
However, I noticed that you DIV element which is the parent of the splitter does not occupy 100% of the page - you can tets this by setting some colored border - the splitter will occupy 100% of its parent and thus, if you want it to occupy the whole page, you should firts configure the DIV. Otherwise, the splitter is taking exactly 100% of DIV.
I recommend to also uinclude the following style in the head:
|html, body, form
| height: 100%;
| margin: 0;
| padding: 0;
and also set explicit height to the DIV (you can do this in percentages, pixels - I hhave set it to 100% in the test page but you can modify the setting according to your needs)
I also reccomend to set the splitter's VisibleDuringInit property to false in ordre to hide the initial resize of the splitter from the end user and to get a better behavior.
I attached my test page to the thread for your reference and I hope it is helpful.
the Telerik team