This post describes the functionality I am trying to replicate: Drag to select when hovering over "background" objects
Trying to use the Alt+Click and Drag selection functionality in Mockups 3 for Google Drive isn’t working in Google Chrome (version 49.0.2623.110 m). I’ve attempted to replicate this functionality in a few different bmpr files, and so far I have been unable to. I’ve also tried in incognito mode with all extensions disabled.
The ability to use Alt+Click and Drag selection works in IE Edge (version 20.10240.16384.0).