I examined the provided code and I suggest to go through the following steps:
1. Remove the FullScreenMode property from all the splitters. Set the outest parent splitter's Width and Height properties to 100% and if it is nested in another element, e.g DIV, set explicitly its height, too (do this for all the parent elements of the outest RadSplitter).
2. Put the following style in the page's head section:
html, body, form
3. Do not set any sizes to the nested splitters if they are directly nested in a RadPane - when a RadSplitter is directly nested in a RadPane, it automatically resizes itself to occupy 100% of the parent pane. If you set width, height, fullscreenmode, the splitters will make a lot of calculations to resize themselves and this is not necessary and slows down the performance significantly. If the nested splitters are not directly nested in a RadPane but in another element, set their sizes and also set the ResizeWithParentPane property to false.
4. I also noticed that you have only two radPanes in the outest splitter and you have set Locked=true for both of them - this is invalid - having only one of them locked is enough to not let the other one be changed.
For your convenience I attached your modified page (I removed the custom skins settings in order to visualize the splitters).
the Telerik team
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