Okay, this threads now involves the original bug report and an issue about emojis in Wine. Please try not to mix topics in one thread in future, it makes it really difficult to track issues.

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I just opened a chat window, actually set the font to Arial and also to Lucida Sans Unicode and in either case the effect is the same. If I put any unicode emoji, such as 😀 in a message, it just shows up as two empty squares.

There are several issues here. First, you need a font that supports emojis. On Windows, the only installed font I have found, so far, that supports emojis is Segoi UI Emoji. Second, the fallback/font-linking code is ridiculously complex and took years to tweak to make it work reasonably well. I am not going anywhere near it just to try to get it to work under Wine :-] Third, see here for a Wine bug report that sounds like your issue.