I already answered your other thread and for your convenience and for others who might encounter the same issue I pasted my reply below:
The FullScreenMode property is obsolete and it is not recommended to use it.
When you have the RadSplitter configured in percentages, all its parent elements including the body, form and html should have their size explicitly set either in percentages or pixels in order to let the splitter calculate the size it should take. This is not new and I do not know why it worked in your previous version - you can see a sample demo explaining this below:
This being said, you will get the desired behavior by adding the following style to your page:
| <style type="text/css">
| html, body, form
| height: 100%;
| margin: 0;
| padding: 0;
Do not forget to set height to all the other parent elements if there are such (e.g DIV, TABLE, TR, TD, etc).
On a side note, I also suggest to set VisibleDuringInit=false for the splitter - this will hide the initial resizing of the splitter from the end user and you will get a better behavior.
I hope that my explanations and suggestions are helpful, let me know how it goes.
the Telerik team