You can open a second sidebar on the right side of the editor, and use both side bars to arrange the Designer panels.

If you want to resize the editor, drag the handle in the middle of each border to make the editor pane larger or smaller.

Click the handle to close the sidebar, and maximize the editor to this side of the application.

In the BPMN editor, you have another option:

To gain more space for modeling, you can hide the whole execution pane by clicking the icon from the model toolbar.