WMS parent layer added via QGIS Browser doesn't use default style

[pathmapper]

What is the bug or the crash?

WMS parent layer added via QGIS Browser doesn't use default style.

Steps to reproduce the issue

1. In QGIS Browser, add the following WMS connection: https://alkisservices.krzn.de/WMS_ALKIS_viersen/ALKIS
2. Open the debug panel to observe the network requests
3. Add layer Viersen Amtliche Basiskarte 1:2500 via drag and drop to the layers widget
4. Inspect the GetMap URL, it contains parameter STYLES=gelb but it should be STYLES=

Versions

QGIS version	3.21.0-Master	QGIS code revision	8af1fae
Qt version	5.12.8
Python version	3.8.10
GDAL/OGR version	3.0.4
PROJ version	6.3.1
EPSG Registry database version	v9.8.6 (2020-01-22)
Compiled against GEOS	3.8.0-CAPI-1.13.1	Running against GEOS	3.8.0-CAPI-1.13.1
SQLite version	3.31.1
PostgreSQL client version	12.8 (Ubuntu 12.8-0ubuntu0.20.04.1)
SpatiaLite version	4.3.0a
QWT version	6.1.4
QScintilla2 version	2.11.2
OS version	Ubuntu 20.04.3 LTS
Active Python plugins	sagaprovidergrassproviderMetaSearchprocessingdb_manager

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

From the WMS 1.3.0 spec:

7.3.3.4 STYLES

[...]
A client may request the default Style using a null value (as in “STYLES=”).
[...]
If the server advertises several styles for a layer, and the client sends a request for the default style, the choice of
which style to use as default is at the discretion of the server. The ordering of styles in the service metadata does
not indicate which is the default.

From https://alkisservices.krzn.de/WMS_ALKIS_viersen/ALKIS?SERVICE=WMS&REQUEST=GetCapabilities :

[elpaso]

I think I can fix this, but there is still a different behavior between the browser and the data source manager dialog: in the dialog you have an individual entry for each style while in the browser there isn't. But I guess that it would be a different issue, probably a feature request.

[pathmapper]

Just tried the fix, works great, thanks @elpaso 🎉

I guess that it would be a different issue, probably a feature request

I think so too.