This is probably something that will be obvious to a more accustomed eye (I rely on it).
I am overlaying nils weidmann's cshapes (http://nils.weidmann.ws/projects/cshapes/shapefile.html), from geoserver via wms, over google maps, at https://www.degrit.com/hotw (does not work with Safari yet, Edge who knows).
While it would seem obvious that something is wrong with the projection while zoomed out, if you zoom in all borders overlay the map perfectly at any longitude/latitude.
I wish I had a better mastery of the jargon to formulate a better question, but... what gives ??
As a layman I would have thought the misalignment to exist at all zoom levels.
- it looks like it's trying to wrap the layer around a spheroid as you zoom out
- zooming out in geoserver's Layer Preview does not distort the layer, either with EPSG:900913 or EPSG:4326 (which presumably means it works fine in OL ? I'm going to put up a quick test page and see, but frankly it only makes things more confusing).
- I'm sorry to say the quick test page (https://www.degrit.com/hotw/ffs.php) with OL works like a charm, without having to even specify a srs.
Edit re: hold/closure. While the original circumstances do no longer exist, I think if I post a couple of screenshots of the problem (which I can revert to easily) they, together with the self-answer I am going to accept as soon as it lets me, may spare some other poor soul a whole day of frustration.