With the current app layout, the panel on the right is always the one with a dynamic width. This is fine with the current default layout, but a problem when the user moves the panels and the last one is no longer the editor.

In that case it leads to unexpected behaviour, such as the Outline plugin panel expanding horizontally even though it would be expected to have a fixed width.

To fix this, we should only give the vertical panel that contains the editor a dynamic width. All other panels should have a fixed width.