Menus: Filter orphaned locations on theme switch

[obenland]

With menu mapping in place (#39692), it is possible to get unexpected mapping results after nav menu locations have been removed. If the nav_menu_locations theme mod still contains data for that removed location, and the slug happens to be part of the group of slugs that are being mapped, it is possible to have that outdated value mapped to a new menu location when switching themes.

I first noticed it when debugging the menu preview in the customizer, which in its call to wp_map_nav_menu_locations() would always return the menu locations of both old and new themes.

[obenland]

@afercia Would you mind testing menu mappings again with 42121.diff​?

[afercia]

@obenland this was what I got during our testing on Slack:

with the patch applied there's a difference in what get_nav_menu_locations() returns (just 2 locations instead of 3) but the fallback menu is still there for me:

[obenland]

Right, with the patch it will now only return the menu locations that are registered. I don't know what the issue is with the fallback menu, but it seems unrelated to menu mapping.

[obenland]

In 41811:

Menus: Limit mapping to registered locations

Weeds out orphaned locations, so their information will not continue to be mapped to future themes.

Fixes #42121.

[afercia]

Agreed with @westonruter to reopen for investigation on the edge case illustrated in the screenshots above, see also conversation on Slack here: ​https://wordpress.slack.com/archives/C0381N237/p1507235780000471

[obenland]

@afercia Could you be a bit more specific as to what I should be testing for? Switching back and forth between 2016 and 2017 with empty menu locations doesn't produce the results you described in Slack. Both menu locations remain empty, no theme displays a fallback menu.

It would be great if you could provide steps to reproduce this from a state where theme_mods for both themes have been removed.

@westonruter Have you been able to reproduce?

[afercia]

@obenland I guess @westonruter wanted to reopen for investigation exactly for the reason that seems there's an edge case that it's difficult to reproduce. I have been able to reproduce it again randomly switching themes, even to 2013 back and forth. I can provide a dump of my local environment database if needed.

[obenland]

Replying to afercia:

I have been able to reproduce it again

What is it in this case? Assigned menu locations when there weren't any or falling back to default menu when it shouldn't?
As much as I'd love to help, I'm not sure how really without knowing what to test for. Randomly switching themes hasn't done anything for me since [41811].

[obenland]

@afercia Any update here or can this be closed?

[afercia]

@obenland I've tested again and wasn't able to reproduce the issue any longer. That doesn't necessarily means we've clearly identified the root cause :)

[obenland]

Let's close it out and open new tickets for reproducible bugs.