I use a mouse trackpad about 75% of the time. One thing I notice missing when viewing large images is the ability to scroll left to right and right to left horizontally on a large image with a trackpad. Scrolling vertically works perfectly fine on a mouse wheel and trackpad. My workaround for this at the moment is to simply drag the image horizontally with the hand tool which rather a bit inelegant on a trackpad -- but it's actually okay on a mouse. There are only a few things that I find lacking to the viewer in comparison to irfanview (e.g. very fine grain scrolling) as a daily image viewer, and the missing horizontal scrolling with a trackpad is one of them.
Are you saying your trackpad supports a horizontal scroll gesture that doesn't work in Opus?
Ah, yes. I know many trackpads nowadays do have a form vertical gesture scroling enabled by default (usually edge rotation scrolling of some kind). The actual horizontal type of edge scrolling is something I haven't always seen--or enabled in the software by default so you have to dig in the settings to find it. Maybe I'm just the first one to notice as few people actually use this feature as it's hidden in many trackpads by default? Probably. I can't be sure.
I'm sure Opus should be able to support this as horizontal edge scrolling works perfectly while navigating the folder tree and the lister display. It's just the viewer where it appears to be absent (I've checked preferences/settings and couldn't find an option to enable it).
This works fine on my Logitech trackpad.
It also responds to horizontal scrolling via Tilt Left / Tilt Right on my Roccat mouse.
The viewer will respond to both horizontal-mousewheel (WM_MOUSEHWHEEL) and to scrollbar messages (WM_HSCROLL - at least as long as the scrollbars are on, but I think even if they are off) and the left and right cursor keys. One of those should be what the trackpad drivers are sending. Sometimes drivers do strange things, though, or send the messages to the wrong place.
Sometimes drivers look for scrollbars to send the messages to, so whether the scrollbars are turned on or off in the viewer may influence what happens.
I've also seen drivers (Logitech) had very different behavior for vertical and horizontal scrolling, for reasons unknown. (Possibly dating back to when Windows only did one and not the other, so they had a separate codepath for the newer one.)
It is rather strange. On my end it's only both when using the viewer as a pane and even in standalone mode that this function is absent in opus. Although I probably shouldn't be entirely surprised given that edge scrolling in some programs I have also do not work, but only a few. It actually might be my trackpad drivers. I've only noticed this strange inconsistency after upgrading to Windows 10 and god knows I've already reinstalled and updated and reverted drivers more than two dozen times. I thought Opus was the issue here as horizontal scrolling does work in the folder tree and lister with just the exception of the Viewer.
I forgot to mention: I'm using the in-built trackpad of my Samsung laptop with synaptics pointing device software. Unfortunately, I can remember having made many past inquiries and visiting many forums and trying out their solutions (specifically vertical scrolling in some programs not working e.g. the free OneNote [but it works with my paid for office 365 OneNote]) but nevertheless not helping resolve it completely at all. The latest driver update for this specific trackpad available is 2011... so well. Manufacture support is nil at this point.
Windows 10 changed how mousewheel messages are routed by default, which may affect the touchpad gestures as well.
Unless you tell it not to, Windows 10 will route mousewheel messages to the window under the mouse pointer, not to the currently active window like it used to. It's a strange change for them to make after so long, and causes problems with some things (e.g. standard drop-downs can now behave unexpectedly in a couple of ways). You can turn off the new routing, but if you do it unfortunately breaks scrolling the Start Menu, because Microsoft no longer test things properly. 
Not all programs support WM_MOUSEHWHEEL messages (WM_MOUSEWHEEL is common but the horizontal variant is less well known), but most support WM_HSCROLL. It depends which the trackpad is sending, although both should work in Opus. If it doesn't work with a few things that that might indicate it isn't generating the messages in quite the standard way, unless it's the focus/routing problem I just mentioned.
Hmmm, if by fixing this routing manually I lose/break scrolling in other areas, might as well not. And here I thought Microsoft and Samsung were just trying to force me to buy a new computer [no thanks to the free Windows 10 upgrade] and renew my yearly office 365 subscription which is due in three months yet again. Hah! I still suspect there is some something to that -- a kernel truth of sorts. I think it's time to replace this laptop soon, anyhow. Thanks for the very detailed explanation, though, Leo!