The default chat frame has this feature. If I was away for a while, I just usually scroll up the chat very very slightly (1 tick as mentioned in the title), and the whole history that can fit in the chat frame, shows up.
This would be a nice option to have, even if it wouldn't be a feature by default.

Not sure I'm understanding this correctly, but Glass already does this. When you scroll up, all chat messages that fit within the window are shown.

Scrolling didn't work properly for me, so I couldn't tell that this was a feature. Sorry about this.