This is a problem I see in every web browser I use: the horizontal scroll bar in the document pane isn’t tied to the bottom of the actual pane. I can resize the pane for a larger preview, but the horizontal scroll bar stays anchored to the bottom of the last item in the list. I’m uploading a screenshot to illustrate my point.
The scroll bar is part of the table control and unfortunately with this control you cannot force it to be bigger than the number rows in the table.
Fair enough. As a design suggestion then, I propose you begin the horizontal splitter immediately where the scroll bar ends in such cases to avoid the eye-puzzling gap and wasted space.