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I'm using Mac OS X Yosemite (latest) with the latest Safari (but I've also noticed some of these problems with Google Chrome). These things are so fundamental to basic data entry and the user experience it's hard to know how they'd pass alpha testing, and persist for years, but they have.
A great example of a functional well designed forum interface is StackOverflow.com. It even has a super convenient mark-up language. It's geared more to code. But at least it's a well designed/implemented interface that's solid and could provide a reference.
Some sort of blend of the chess.com/stackoverflow.com interface would be close to perfect.