I already suggested something similar to this
here and the idea was sadly rejected on the grounds that it's past the deadline for new features being introduced to NeoLemmix. Namida has plans to release a "final" version of NL and wants to wrap up the process of adding new stuff, understandably.
However, a quick search led me to
this topic, which was never actually resolved as such and which seems to be suggesting a similar thing. From here, I discovered that NeoLemmix has a
Hold-To-Scroll function which can be assigned to a hotkey.
Try this:
1 Open NeoLemmix
2 Click the cogs card (or press F3) to open the settings menu
3 Click "Configure Hotkeys" - this opens the hotkeys dialog
4 Make sure "Show Unassigned Keys" is checked (at the top right)
5 Choose any unassigned key (it will say "Nothing" to show that nothing has yet been assigned)
6 Use the dropdown list on the right to find the "Hold-To-Scroll" option, and assign it to your chosen key
Now, close the menu dialogs and launch a level which requires horizontal or vertical scrolling. You will notice that by pressing and holding the hotkey, you can use the mouse to track around the level. I wonder if this solves your issue?