Downloaded 2.7.8, after mounting the disk image, I try to open the package. After about 5 seconds I get a 'verifying' progress bar that does not progress and does not go away, even after 5 minutes.
OS: 10.11.4 (15E65)
Either keep waiting or file a radar at http://bugreport.apple.com. I'm not sure what "verifying" progress bar you refer to. The only verifying progress bar that I'm aware of in this entire process is the one verifying the disk image as part of mounting it. Once it's mounted, I don't think there are any more verification progress bars during the install.
*** This bug has been marked as a duplicate of bug 94287 ***