If I am understanding your problem correctly, you want to have a SplitLayoutPanel to occupy maximum space of the parent element. For this you can use the percentage property of width like,
SplitLayoutPanel slp = new SplitLayoutPanel();
slp.setWidth( "100%" );
And in the next step you want to add child element into the SplitLayoutPanel and it should occupy maximum area. Then set each child width to 100%
Widget child = // Instantiate any type of widget
child.setWidth( "100%" );
slp.add // Use proper add method and add the child to SplitLayoutPanel
I did not understand what did you mean by dragging is very jumpy. Did you mean Splitter size is too small that you cannot find the splitter. If thas the case, you can increase the splitter size by passing it in the constructor like
int splitter_size_in_pixels = 5;
SplitLayoutPanel slp = new SplitLayoutPanel( splitter_size_in_pixels );
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