[MapProxy] Problems with TMS and grid _skip_first_level

[G. Allegri]

Hi,
I'm using OL 3/4 for an application where a TMS layer is defined:

new ol.layer.Tile({
   source: new ol.source.XYZ({
      url: "
http://localhost:8080/tms/1.0.0/mylayer/webmercator/{z}/{x}/{-y}.png",
   })
})

(the inverted "y" in a XYZ layer makes it behave as a TMS layer).

The map is EPSG:3857 and the layer source (WMS service) is also EPSG:3857

This OL app works perfectly with other cache tools, like mapcache, with a
standard mercator grid.
When I use it with MapProxy things get wrong becasue TileServiceGrid skips
the first level for GLOBAL_(WEB)MERCATOR :
https://github.com/mapproxy/mapproxy/blob/master/mapproxy/service/tile.py#L381

If self._skip_first_level is forced to False everything goes fine.

I noticed that even in you OpenLayers 2 Demo you start resolutions
from 78271.516964, which is the second level of the the classical mercator
grid.

My questions are:

 - why is the first level skipped?
 - how could I define a GLOBAL_WEBMERCATOR grid without skipping the first
level? I don't want to force it at level of (internal) code, obviusoly.

Thanks so much,
Giovanni
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