Comment 39 for bug 1653351

Ok ok, I'll make an add-on to fix this (not that it bothers me personally). :)

Here it is: https://addons.mozilla.org/en-US/thunderbird/addon/deselect-on-delete/

For the curious, here's the code to do this (though you could obviously open up the .xpi to see it too):

FolderDisplayListenerManager.registerListener({
  onMessagesRemoved: function(display) {
    display._nextViewIndexAfterDelete = null;
  },
});

I'll leave this bug open just in case this ever gets into Thunderbird proper, but the issue should be resolved for most users now.