Prevent server-side rendering of language switch

[saschaszott]

References

n/a

Description

The language switch dropdown is now wrapped in a @defer block. As a result the language switch is no longer part of the server-side rendered HTML. It is is only rendered in the browser / client-side after hydration.

Instructions for Reviewers

The language switch is only useful to interactive / human users. Nevertheless, SSR renders the full dropdown into every page. With the default configuration (35 languages), this adds about 7 kB of markup to every SSR response. After this change, only the empty host element is left, about 136 bytes.

Angular never renders @defer blocks during SSR, and it renders them on the client after hydration.

The default custom theme reuses the base template and is therefore covered as well. Custom themes that provide their own lang-switch.component.html are not affected and keep their current behavior. I'm not sure if this note should be added to the release notes.

The language switch button now appears shortly after the page has loaded in the browser, so neighboring header icons may shift slightly while the page is loading.

How to Test

0. Make sure your test instance contains at least two active languages
1. Build and start with SSR: npm start.
2. Open any item landing page in a browser:
   • The globe button in the page header appears after the page has loaded.
   • Opening the dropdown and switching the language works as before, with both mouse and keyboard (Enter).
   • The browser console shows no hydration errors (NG05xx).
3. Check the SSR output of the item landing page: the element ´ds-base-lang-switch should be empty (<!----> only). There should be no ngbDropdown and no .dropdown-item entries.

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