I strongly disagree with your claim.
To download all content click on What happens when a topic has over 1000 replies? - general - Discourse Demo. If the click happens to perform a partial download only try the browser reload button.
With unread text dragging the button of the browser scrollbar to the top moves 20 posts at a time on this thread e.g. from position 737/1000 to > 720 > 700 > 680 … Dragging to the bottom skips forward e.g. from 325/1000 > 345 > 365… With read text browser scrollbar reverts to default. Verified behaviour with NVIDIA 260.19 Issues - #20 by Lord_Emsworth
Users annoyed by the unread text feature of the browser scrollbar may e.g. press and hold the Page Down key until reaching the end of the thread.
Note: According to the stats at Profile - karlmistelberger - openSUSE Forums I read some 11.1k posts in the last 90 days. Nonetheless I was never affected by the issues reported by @mrmazda.