This is also a problem with me too, in FB, YouTube, and other websites that implement heavy JavaScript. Safari truncatesd the Headers of websites, usually after a javascript tiimeout
Although Safari is marketed as super fast, it is the slowest browser i have on my Mac (have 5 different browsers), and is also the most deprecated as far as playing videos on sites like cnn, the guardian, the sun, fox and so on. It just cant play them. For me its not an option to not use safari because its the only browser that lets me use text replacement shortcodes to fill in text strings that i use repeatedly when i am authoring stuff on the web, and it saves me a lot of time. Otherwise i would consider Safari to be totally deprecated due to massive incompatibilities with many modern websites. Having said that, i also do not blame Safari, rather, many of these 'modern' websites, like fox and time and cnn and the guardian and so on, because they dont try to make their websites compatible with all browsers. it should be the job of the web designer to make compatibility, not so much the browser developers . Its definitely to do with the jasvascript and flash like code found in websites like facebook that cause this truncation of headers. But i must add that my wordpress sites also get their 'heads chopped off' by Safari when i leave the tab open without using it for a while. It seems to be a timeout issue involved, as it happens after a time, not instantly. Facebook is also inconsiderate with its mega massive code (BIG DATA mining of User behaviours)
Anyway its definitely unfixable and i wouldn't hold your breath waiting for Apple to sort it out, because they don't really care about these kind of issues, and blindly brag on every Keynote about how good it is (lol really blind arrogance)
Its like the brags about the new features in Mail app for iOS.. but never mentjon that the OSX mail app cant even default text font size, and when reply to a mail and make a new line, the text changes from lets say 15 px to some tiny 8 px.. you cannot control font sizes at all and its like a 1998 mail editor.. no improvements at all to that since about 2010.
Safari just cant display all websites, and Apple doesn't seem to be aware or care.