is_admin() conditional tag is not true on the Customizer page

[ScottSmith]

Steps to Reproduce:

1. Add the following code or similar to any page of the theme to test whether the is_admin() conditional tag is true or false.

<?php if ( ! is_admin() ) {
     echo "You are viewing the theme";
} else {
     echo "You are viewing the WordPress Administration Panels";
}?>

2. Open theme customizer

Expected: Text should read "You are viewing the WordPress Administration Panels" because the theme customizer is an admin panel.

Result: Text reads "You are viewing the theme," when in fact, you are in an admin panel.

[SergeyBiryukov]

Related: #23509

[johnbillion]

Suggesting close as wontfix, and in favour of introducing a new conditional in #23509. If we start returning true for is_admin() on the front end then it'll mess with plugins that control access to the admin area and we'll get all sorts of code being executed when it shouldn't be.

[nacin]

This is also by design, to make sure themes checking is_admin() were either loading or not loading the proper stuff in the controls panel (considered the admin here) and in the preview frame (considered the frontend). This was designed to work with all themes without modification, which was quite a bit of a feat in itself.

[badfun]

Was just about to report the same 'bug'. If this was by design, then I suppose the problem is really communicating that the customizer preview is really front-end, and not in the admin, even though it is, well, in the admin.

is_customize_preview() is great of course. thanks!

[DrewAPicture]

I would suggest supplementing the inline docs for is_admin() to mention this case. The Customizer is now consistently being described as a "bridge" between the back- and front-ends, so I think that's something worth mentioning by default for is_admin() in ​its article in the code reference.

Removing the needs-codex keyword as we're really trying to shift effort to inline docs and the code reference over the Codex.