Load the template list within the Site Editor without page reloads #37075
Description
Follow-up to #36379
What problem does this address?
In WordPress 5.9 Beta 1, the template list meant to allow for quick navigation between templates within the Site Editor is not loaded on the same page as the Site Editor; there is a page reload both when navigating from the Editor to the template list and when clicking on a template and going back to the Editor. This implementation is not cohesive with the rest of the Editor, which works as a single page with no reloads (e.g., when entering the Template Editor in the Post Editor, or the previous Site Editor sidebar implementation), and degrades the user experience with blank loading screens with no loading indicators.
The example below demonstrates the flow running a brand new local site with WordPress 5.9 Beta 1 and TT2:
Grabacion.de.pantalla.2021-12-02.a.las.19.51.18.mov
What is your proposed solution?
The template list design represents a good first iteration, so it should be researched how feasible it is to load the template list inside the Editor for WP 5.9, even if it is as a modal window but always without leaving the Editor. One of the constraints considered in the current approach is the need for a specific URL for the template list (context in #29630), although it is not a requirement right now and should not be considered when considering this for 5.9.
Because of the time constraints of fixing this during the WP 5.9 beta, if the above doesn't seem feasible for this release, we should aim at enabling back the original W menu in the Site Editor and polish the design to focus on implementing the above as the next iteration for WP 6.0.
This issue is a follow-up to #36379, a blocker of the original WP 5.9 Beta release. Contributors worked hard in a crunch to have an MVP ready for the revised Beta 1 date, and this issue aims to fix the UX concerns found as a direct result of the beta testing of this MVP and should therefore be addressed for WP 5.9.
To be mindful of the adjusted WP 5.9 timeline, we should make a decision on which approach to take during Beta 1.