The undocumented SPACESWITCH system variable

The undocumented SPACESWITCH system variable in AutoCAD can be used to allow or forbid switching between paperspace and model space within a layout viewport with a double click. This tip can be useful if you have annotations in paper space and don't want to unintentionally activate the floating model space. It was recently added to the Autodesk Knowledge Base so now it's at least somewhat documented.