It appears the Basemap Gallery widget does not support adding basemaps with different coordinate systems. This is a known limitation according to the documentation for both the Portal/Online version, as well as the Developer version of the WAB.
Has anyone else had a need for different coordinate systems within the basemap gallery and are there any workarounds for this (specifically around the Developer edition)?
Basemaps with different coordinate systems work just fine under the Portal/Online map viewer's basemaps gallery widget. Don't understand why the limitations in Web AppBuilder...
When you load a map in a web page it loads the basemap first. All of the levels of detail (LOD) and extents are now set for the map. If you now change basemaps then your new basemap probably does not line up with the old one because of the different LOD and extents. It probably still adds it to the map but in a place where it is not usable and probably hidden due to the LODs. There is no projecting a basemap on the fly because it sets parameters for the entire map. Your data will project on the fly because the map now has a projection to convert from and project to. My guess as to why AGOL works is that it reloads the entire map object upon switching. Best guess.
What you are saying makes perfect sense and I am aware of the different mechanisms behind changing to a basemap with a different coordinate system.
What I don't understand is why reloading the whole map object (if this is what happens in Portal/Online map viewer) is not feasible in the WAB Basemap Gallery widget...or at least this is my assumption due to the limitation on hand
Would actually be nice if someone from the WAB development team can shed some light on this.
Are there any plans to add the ability to have basemaps in different coordinate systems in the Basemap Gallery widget??