Are we fine with nested scroll bar? I’m not. I really want to avoid nested scrollbar if we have another option.
Topic list and topic editor both have scrollable categories select bar. In there, if scroll hit top or bottom,. whole current page start scrolling. It’s quite nervous keep away from top or bottom especially dealing with poor sensitivity mouse wheel or touchpad.
I think selecting category is active user action. Not like new topic, reply, notification or some other passive actions. Thus we could be more aggressive on space usage. Because they actively select categories button, not gonna excused even take whole area for category popup. We should show them quick and easy at glance because they just clicked button for that reason.
IMO, center aligned, big square box (e.g. 80% max, 16:9 ratio) would be fine. It maybe better using pagination instead of scroll bar if there are so many categories. It’s almost like image view popup box.