This is not only the namespaces selected and saved by the users but also the list of default namespace searched by default.
Currently when the extension is enabled you can encouter a strange behavior that looks like a bug:

• Enable the AdvancedSearch beta feature
• On any page
• Search using the box top right (top left if rtl)
• you'll see results for multiple namespaces (either user defaults or wiki defaults)
• hit the search button again (without changing anything)
• you'll see results for NS_MAIN only

thanks @dcausse! I created another issue for that here: T184589: Keep namespace selection when entering advancedSearch

Remember that some work was already done in https://gerrit.wikimedia.org/r/#/c/410189/

API reference for the browser tests (for setting the namespaces): https://www.mediawiki.org/wiki/API:Options

@Lea_WMDE Question regarding the interaction with the namespace presets.
Imagine

• I load a the search form with my default namespace selection (say 4 namespaces)
• check the "all" namespace preset
• un-check the "all" namespace preset

Which namespace(s) should be selected?

@Charlie_WMDE what do you think?

@Lea_WMDE @Pablo-WMDE

Hm, the current UI allows for deselcting all namespaces but when conducting a search it will just select (Main) automatically even when the default is another combiantion of namspaces.

I suggest we do the same. un-checking "all" reverts back to only the main namespace to be selected. Reverting back to default namespaces makes little sense since what un-checking suggests is the removal of everything or in our case as much as possible since at least one namespace needs to be selected.

@Pablo-WMDE I was wrong. I'm not sure what I did last time when trying it out but I just tried it again and when removing all checkmarks and then execute the search it searches in my preselected default namespaces. I'm really sorry about this.

In this case I suggest we stick with the way it is currently done. un-checking "all" should search in the default namespace selection. My only problem with this is that we don't give the user the possibility to visually remove all selected namespaces. Worst case scenario would be if a user has all namespaces selected as default and would then always have to remove them manually one by one. It would make more sense to visually remove them all and when the search is executed search in the default namespaces and add them back to the namespace field. Is this possible?

@Charlie_WMDE @Pablo-WMDE For the MVP let's go with a simple way first. I suggest we go with the current behavior now (i.e. default namespaces) and discuss more complicated things later?

un-checking "all" should search in the default namespace selection.

This would require for a change. Currently advancedsearch effectively does not allow you to remove all namespaces. The unchecking of the "all" preset tries that, but there is a dedicated logic in place (SearchModel.prototype.setNamespaces) that ensures that when no namespace would be selected, at least one - "MAIN_NAMESPACE" - is indeed selected. This is also played back to the user via UI. Consequently, when submitting from that state, the fallback to the user-specific default namespaces (which seems to be based on "no namespaces were search in") never happens, and the search is performed inside "MAIN_NAMESPACE" only.
To arrive at the described state and be consistent UI-wise, when un-checking the "all" namespace list should be updated to the list of default namespaces - but not sure if this is intuitive...

/cc @Lea_WMDE @Charlie_WMDE

Follow-up (after discussion with @Lea_WMDE) in T187911

@Lea_WMDE for the MVP I'm fine with working with what we have. I will note this down so we don't forget in a future discussion since I think this will need some more thought.