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I installed telegram through the official telegram-desktop package. While the icon works when searching for the application, it doesn't in the preview given by alt-tab or in the tray using Gnome. I fixed this issue by changing the StartupWMClass in /usr/share/applications/telegramdesktop.desktop from "TelegramDesktop" to "telegram-desktop".
I'm mostly posting this so others facing the same issue can find a solution. If anybody's interested: Do you know if this is a good solution? Should I be posting a bug report to either telegram or the maintainers of the package?
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