I posted 1st step, which supports MacOS horizontal scroll (via 2 fingers on trackpad)
to http://bugs.freepascal.org/view.php?id=29972
Pls apply. CudaText uses it. Now it scrolls horz, it is 1st app on Mac which can scroll horz via trackpad (not by scrollbars).

This step added setting of ssExtra1 flag to Shift(TShiftState) in OnMouseWheel.
If horz scroll, flag added. If vert, flag not added. ATSynEdit handles this flag.

Next step needed for other Laz users.
Others want that horz scroll works in all LCL controls(Memo/Treeview/etc). So maybe needed to handle ssExtra1 in Memo/Tree/etc. On maybe other: needed to force some methods on LCL level w/out changing LCL components