HwKeeper
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Previously, when opening a message with a YouTube video, a preview image of that video was shown, but now it's just a black square with the text "click to view".
This is done for maximum privacy and protection from tracking, and so that there's no need to show the annoying consent dialog for third-party cookies required by law.
That is, until you click that button, YouTube knows nothing about you; when you click it, it starts tracking you.
It could be made optional to disable tracking protection in the settings, but I don't want to disable it globally, since I don't want to introduce a cookie consent dialog.