This idea is meant to increase the consistency and suitability of the use of the middle mouse button (MMB) and scroll wheel (SW) in navigation (scrolling, zooming and panning).
Current Usages of the MMB and Scroll Wheel
The SW can be used to both scroll up and down and for zooming (common in 3d graphics software and 2d graphics software when the Ctrl-key is held down).
The MMB can be, and mostly is used for panning. However most document-software doesn't use the MMB (for example libre office, but GNOME documents does).
Zooming, scrolling and panning could be smooth (like in touch OS's, GNOME documents), but is often not smooth (especially common in 2d graphics software).
The idea is to split up the behavior in two distinct cases, document-behavior and image-behavior. In documents it's very common to scroll up and down, compared to image-viewing, where scrolling up/down doesn't make much sense, but where zooming is more necessary. So there is a natural difference here that the user easily could distinguish. More specifically this means:
- SW = scrolling up/down
- Ctrl+SW = zooming
- MMB = panning
To be used in software like: LibreOffice Writer, document viewers and web browsers.
- SW = zooming
- Ctrl+SW = this otherwise unused combination could be used for zooming in/out to typical "states", for example: 100%, show all, selection or page (in the order of the actual percentage because the actual size of these could vary)
- MMB = panning
To be used in software like: 2d graphics software, image viewers and map viewers.
Also I would suggest that all panning, zooming and scrolling would be smooth, however, in 2d graphics software I could see a problem with this.