When using the current WebKitGtk (even in the MiniBrowser), hangouts.google.com chat interface does not render correctly. After logging into that page, if you start a chat conversation with a contact, the chat window renders wrong. The portion of the chat window that has buttons to start a video chat is invisible, and just shows the background image of the page. The same is true for the bottom part of the chat interface where you type. It is invisible and shows the main page background image instead. Digging through the Inspector, I can get the top bar to show up if I remove a CSS rule that is setting the z-index to 2, but I can't find what is causing the problem on the bottom one.
Further testing shows the same behavior on the GMail inbox page. If you bring up GChat windows, you might see the same thing. It seems to only happen in certain cases though. The most reliable way to reproduce is to maximize the browser before visiting hangouts.google.com. Then sign in, and then open a chat window.
Further testing reveals this to be completely related to the display of emojis. If there is supposed to be an emoji visible in the chat window, the entire page display gets messed up in strange ways. Seems to be related to Google's recent change to change the PNG that holds all the emojis from a grid to a single column. The png is now something like 28px X 33000px. I believe this is too tall for WebKit to handle correctly, but I can't figure out why.
*** This bug has been marked as a duplicate of bug 168964 ***