Can you maybe post a video showing what's happening? It's hard for us to understand what you mean without seeing it in action.
Does it happen if you disable other WordPress plugins and go back to a basic WP theme (wondering if another non-sp JS script isn't affecting the page).
We're in the process of testing by deactivation of plugins etc.
In the meantime, this is a video that hopefully shows the issue more clearly:
If this can help with identification of the problem, please let me know so I can pass on to my colleague who is trying to fix this.
Thanks for the video - I see what you're talking about. I haven't been able to duplicate that here so far, even with very long forum pages or multi-page posts. So as much as I hate to point fingers I have to assume for now that there is something else on the page that is contributing to the issue.
But, I would be curious if it happens on a different Simple:Press theme.
We've tracked it down.
Our website host had image lazy load enabled as a site speed enhancement. I'm told when enabled 'the developer console in the web browser is showing images being requested / loaded in as you scroll up the forum thread'.
Our host has lazy load still running but excluding the following image classes which relate to the forum attachments: sfimageleft, spUserImage (could you tell me if that's all of them please) so we've still got lazy load working elsewhere and the forum scroll issue is sorted.
Is there a fix you can look at in a future update?
hmmm...not quite sure what kind of fix we can put in there. If lazy loading is enabled, then every time an image scrolls into view and is loaded, the page will get resized and where you are on the new page size will shift the scrollbar.
But, if you scroll DOWN the page first then all images should be loaded and then if you scroll back up then there's no lazy-loading needed since all images have already been loaded. So the jumping should be minimized then. At least theoretically.
When you click the REPLY link, I think that bypasses the "scrolling down the page" action which means that the images are not being loaded first? So scrolling back up then causes the images to lazy load and adjust the page size. I'm just theorizing of course.