Also, @pod I thought this link you posted was in fact excellent:
Notably, here:
The short version: users expect the back button to take them back to what they perceived to be their previous page. The notion of perception is the key factor here, since there’s often a difference between what is technically a new page and what users perceive to be a new page – which can create discrepancies between where the user expects the back button to take them and where it actually takes them.
At the center of the issue is what users interpret as a new webpage. Generally we’ve observed that if a new view is sufficiently different visually, or if a new view conceptually feels like a new page, it will be perceived as one – regardless of whether it technically is a new page or not. This has consequences for how your website should handle elements like overlays / lightboxes, accordions, form submits, pagination, and filtering.
Thus, it is not a reasonable expectation that moving upward in the topic, which is EXACTLY THE SAME VIEW AS BEFORE, BUT SHIFTED UP, would be considered a “new page” and thus require a press of the browser back button to get back from.
(The other argument you posted, that a near-fullscreen image on mobile seems like a new page, is actually one I have heard before on meta.discourse and I am somewhat sympathetic to. For the record.)
But you arguing – at quite some length – that simply moving up 20 posts in a topic is a “new page” and thus requires a “back button”… I’ve literally never, ever, ever in the history of Discourse… ever… had this discussion. Which doesn’t make it bad or wrong, but sets off all kinds of spider sense tingling from my years of software development.
And just to reiterate, because fuck at this point why not, I am also very sympathetic to the argument that our current “move me back to my lowest read position please” button behavior on the timeline is quite shit, and should be improved.