Provided the width of the preview is not reduced to accommodate a scroll bar (an inaccurate preview would be worse than an annoying one), I would very much like to see this improvement. There does not need to be a height to the preview - it can extend indefinitely downwards as it does now, allowing standard scrolling of the whole webpage which people currently like. All that needs to be introduced is a way of adjusting the top of the preview to match the part of the input box currently being edited.
If a straightforward and intuitive way of allowing this doesn't present itself, I would be happy to accept either a button that resets the top of the preview to reflect the edit position, or a tick box that when ticked causes the preview to move with the edit box.
People who don't use the button or tick box will have exactly the same experience as at present, with no new approach to get used to.
If it's simpler to implement/accept, there could be two previews displayed one below the other:
- A box the same size as the edit box, that always scrolls automatically in step with the edit box position (no scroll bar required).
- The standard full preview with no scroll bar, extending downwards as far as necessary to fit in the whole post.
(1) would be small and just show the part currently being edited, while (2) would allow browsing the full post prior to posting.
This alternative approach does not require introducing any scrollbars, so it would not cause problems with the fixed width of the core content. It would simply involve adding an extra type of preview between the edit box and the current preview, and even that could have a tick box to allow hiding it for people who don't find it useful/are attached to the old way.