macOS Safari expert needed to fix tricky video-related bug

We are finishing up a new web site ([login to view URL]) and have found a bug that shows only on macOS Safari. The team does not currently have Mac experts, so I'm hoping the Freelancer community can help. The site is built around the YouTube video player, and includes an iframe that shows external sites. The problem is as follows: After viewing the YouTube video full-screen, window sizing no longer functions in *some cases*. Here is a video illustrating the problem: [login to view URL] It is likely this problem is related to the presence of the iframe because it does not occur when the window is narrow (which forces the site into ‘mobile’ mode without the iframe). Strangely, it also does not occur when the window is very large. The problem also appears to be limited to recent versions of macOS and Safari. This project is initially just to find the cause of the problem, but could be extended to include the fix - for a larger fee of course. Please include the keyword 'banana' in your proposal to let me know you've read it in detail :-)