Each time I try to launch from the Backers Build into Windowed play it starts a download as though I had clicked the download option. Not sure if there has been a change or not but not sure what to do. Using Chrome and Windows 10.
Wow, I think I know what you are talking about. Something like that suddenly happened to my Chrome browser yesterday. Trying to open any SWF file would bring up a notification around the lines of "this file may be harmful to your computer, do you wish to keep it?". I did get around using these steps:
Go to "Settings" >Click "Show advanced settings" > Under Privacy, click "Content settings" >Under Flash, select "Allow sites to run Flash" >Click "Done"
I'm not sure why it started happening, but it worked for me. Let me know if it works for you too.