Turtledemo (python -m turtledemo) has a text pane for code and a canvas pane for the actual demo. The code pane is fixed at about 60% of the width needed to display full-width code lines. If the window is expanded horizontally, all the expansion goes to the canvas pane, where it is mostly not needed or used. Almost all the examples are a fixed size on the canvas, independent of the size of the window on the canvas. 

Having the text pane too narrow makes it harder to read and copy the code than it would be if entire code lines were visible. The default width of the overall window is about 1250 pixels on my big monitor. Many people have more pixels than that. Even on smaller screen, people might like to temporarily allocate more space to the text. So I think the best solution, if possible with tkinter, would be a movable divider.