When I attempt to add a new custom module to a page, I get the following error: 

Uncaught TypeError: Cannot read property 'left' of undefined
    at HTMLDivElement.<anonymous> (ModuleDialog.js?cdv=147:1084)
    at HTMLDivElement.<anonymous> (ModuleDialog.js?cdv=147:390)
    at HTMLDivElement.opt.complete (jquery.js?cdv=147:9285)
    at fire (jquery.js?cdv=147:1037)
    at Object.fireWith [as resolveWith] (jquery.js?cdv=147:1148)
    at tick (jquery.js?cdv=147:8736)
    at jQuery.fx.tick (jquery.js?cdv=147:9317)

Here is more info on what I have.  I created a  compiled DNN9 module from Chris Hammond's templates.  Deployed a basic empty module to DNNDEV.me then built out my module (used ascx code from an older module ~6.2) and everything was working fine.  Switched some old ASP.NET postback stuff to jQuery and JavaScript and changed a bunch of styling to make it more responsive for mobile devices.  Everything was working perfect.  I removed the module - deleted from extensions so I could check the installer.  Debugged a couple of issues with the installer so it would get the correct files in resources.zip.  So, everything seemed fine.  Install worked and all the required files were present.  I can select the module to add but as soon as the draggable box pops on the page everything freezes.

Any suggestions??

To be honest, I don't remember.

Seems like it had something to do with a stylesheet - which doesn't make sense - or a missing javascript file.  What I do remember is, whatever fixed it was a really obscure thing.

Sorry I couldn't be of more help.