#3533614 Added work around for Safari issue where iframe content !== srcdoc attribute value

I don't much like this solution but in my testing it does seem to reliably fix the issue. My best guess is that some kind of optimisation in safari/webkit means that the iframe doesn't load if it's not visible and our iframe swapper is playing about with the rendering of the iframe and in some instances where the browser is running slower, the update to set the iframe to display: block; hasn't completed by the time we update the srcdoc. Or something like that.

Reproducing the issue on Safari seems to be easier when the dev tools are open which leads me to believe there is some correlation with the speed of the browser (dev tools slow it down).

Closes #3533614