Not sure whether this is another Qt bug with QTextBrowser, but VoiceOver cannot navigate the chat history (txvChatWindow) reliably, especially when there is a long message.
When you press the up or down arrow in the chat history and VoiceOver encounters a long entry (for example, a server intro message), it seems to treat the text as multiple wrapped lines, but it only reads the first few words in the beginning. After that, navigation through the rest of the history becomes broken.
Is it possible to present each chat entry as a separate accessibility element instead of one large block of text within the QTextBrowser? That might resolve the issue.