I have to admit, even after using AutoCAD for more years than I care to admit to, I always learn something new (even if it's what not to do!)
Anyway, many of you know me from the Autodesk Online Discussion Groups : http://discussion.autodesk.com as my role as a volunteer facilitator... (or maybe you don't)... but one of the users posted their problem (which I still do not understand why they are doing this, but here goes...)
They stated that they were dimensioning in Paperspace (now known as a Layout) and trying to double-click on the dimension to edit it (which is what I do not understand, but...) they were missing the dimension and then being transported to modelspace (aka: floating modelspace or in the viewport)... I remembered helping someone a long time ago when they were double-clicking on TEXT or MTEXT and it was doing the same thing.
There is a System Variable (undocumented as far as I can tell) called SPACESWITCH. It is only available while on a Layout and when set to 0 (that's zero) it will stop you from being transported into floating modelspace if you miss and object.
The only place I found it listed was a Technical Document:
Unable to enter in to model space of floating viewport
and I remembered showing someone this for editing TEXT, MTEXT on their layout (basically doing the opposite of what the technical doc was intended for) or turning a negative into a positive.