You can change it so that it marks all scrolled posts as read here: Settings shortcut: History > Mark posts as read on scroll
If you enable the FAB, you can arrange your frequent actions so that "hide all read posts" is first. Then, when you long press the FAB, it will hide all read posts. Settings shortcut: General > Show FAB
FAB action has to be set to "more actions" (or whatever it's called). Alternatively, you can just set the action to "hide read", but that will remove your ability to use the FAB for other actions.
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