Hello, I'm an engineer working with Microsoft on the precision touchpad team. Precision touchpad is the class driver for touchpads that ships with Surface devices as well as a large number of Windows Ultrabooks. As of the next version of Windows, paint.net will need to support horizontal mouse wheel in order to receive horizontal two finger panning.
Windows 8.1 and 10 versions until now have utilized an application compatibility layer to support two finger panning in apps that did not natively support WM_MOUSEHWHEEL. Unfortunately, while this worked well for apps like paint.net, it failed for many others resulting in an overall worse user experience. We've chosen to remove this behavior by default, and rely on the apps handling standard messages appropriately. For backward compatibility, we'll be providing a shim that continues to support the previous behavior on the existing shipped version of PDN (4.0.12) and earlier, but any version with a higher major or minor versioning will require this change to continue horizontal panning.
Implementation details can be found here: https://msdn.microsoft.com/en-us/library/ms997498.aspx
Thanks!
-Matt
