x and y are reserved (inasmuch as it's dwl's choice) for coordinates relative to the layout. ox and oy are used for output-relative coordinates. sx and sy are surface-relative. dx and dy are deltas.
figuring this out the first time was kind of a Thing… just don't pay attention to what happens in render and you should be fine.
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* those are underneath the coordinates lx and ly (in output
* Layout Coordinates). If so, it sets the surface pointer to
* that wlr_surface and the sx and sy coordinates to the
* coordinates relative to that surface's top-left corner.
* those are underneath the coordinates x and y (in layout
* coordinates). If so, it sets the surface pointer to that
* wlr_surface and the sx and sy coordinates to the coordinates
* relative to that surface's top-left corner.