Duplicated 404 templates in the TemplatePanel

[Mamaduka]

The issue was initially reported on Github - ​https://github.com/WordPress/gutenberg/issues/33929.

The original "404" template key is lost when we merge "Default template" into the available templates array. The array_merge will re-index/re-number keys with numbering values.

Later, when we merge templates on the client-side, this causes duplicated "404" templates in the dropdown.

Changes array_merge with array_replace. The latter will keep the same numeric keys.

Verification example: ​https://3v4l.org/tlft6

Trac ticket: https://core.trac.wordpress.org/ticket/53898

[Mamaduka]

The issue was reported by @toro_unit.

/cc @gziolo

/cc @gziolo

I don't know this functionalit. @youknowriad, can you help with the review?

@gziolo apologies. I pinged you since this was editor settings related.

Thanks for the review, @youknowriad.

I think I need to swap argument positions so the "Default template" is always the first item in the array.

I will push this change in few minutes.

[Mamaduka]

Thanks for the review, @SergeyBiryukov.

[SergeyBiryukov]

In 51595:

Editor: Preserve the original template keys when preparing a list of page templates.

Previously, the original "404" template key was lost when "Default template" is merged into the available templates array, as the array_merge() function renumbers numeric keys. Later, when we merge templates on the client side, this caused duplicate "404" templates to appear in the dropdown.

By replacing the array_merge() call with array_replace(), we make sure to keep the original numeric keys.

Props Mamaduka, Toro_Unit, youknowriad.
Fixes #53898.

[SergeyBiryukov]

Reopening for backporting to the 5.8 branch.

Thanks for the PR! Merged in https://core.trac.wordpress.org/changeset/51595.

[desrosj]

In 51620:

Editor: Preserve the original template keys when preparing a list of page templates.

Previously, the original "404" template key was lost when "Default template" is merged into the available templates array, as the array_merge() function renumbers numeric keys. Later, when we merge templates on the client side, this caused duplicate "404" templates to appear in the dropdown.

By replacing the array_merge() call with array_replace(), we make sure to keep the original numeric keys.

Props Mamaduka, Toro_Unit, youknowriad, SergeyBiryukov.
Merges [51595] to the 5.8 branch.
Fixes #53898.