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https://bugzilla.wikimedia.org/enter_bug.cgi?product=MediaWiki

Splitting this from bug 34594 comment 5:

The use of mw.user.options.get in the code below should indicate whether the user has or has not enabled the option in its preferences[2]:
----
console.log('$.isEmpty(mw.user.options.values)=', $.isEmpty(mw.user.options.values));
console.log(
    'usebetatoolbar=' + mw.user.options.get('usebetatoolbar') + ';' +
    'showtoolbar=' + mw.user.options.get('showtoolbar')
);
mw.loader.using('user.options', function () {
    console.log('using "user.options" > $.isEmpty(mw.user.options.values)=', $.isEmpty(mw.user.options.values));
    console.log(
        'using "user.options" > ' +
        'usebetatoolbar=' + mw.user.options.get('usebetatoolbar') + ';' +
        'showtoolbar=' + mw.user.options.get('showtoolbar')
    );
});
----

Nonetheless, I've tested it on Portuguese Wikipedia[1], which is currently running MW 1.18wmf1 (r109351), using Google Chrome 17.0.963.56, and the result is inconsistent:
On each of the following cases
1) Only "Show edit toolbar (requires JavaScript)" enabled
    Note: Old toolbar was loaded, as expected
2) Only "Enable enhanced editing toolbar" enabled
    Note: Enhanced toolbar was loaded, DISRESPECTING the user's choice of disabling "Show edit toolbar (requires JavaScript)". This was described on bug 30795, and I believe it justify marking bug 19786 as WORKSFORME.
3) Both options enabled
    Note: Enhanced toolbar was loaded, as expected
4) Both options disabled
    Note: No toolbar was loaded, as expected
the result is always the same:
----
$.isEmpty(mw.user.options.values)= true
load.php:1usebetatoolbar=null;showtoolbar=null
load.php:1using "user.options" > $.isEmpty(mw.user.options.values)= false
load.php:1using "user.options" > usebetatoolbar=1;showtoolbar=1
----

So, after the module "user.options" loaded, mw.user.options.get(...) isn't returning any useful information about the user's choices regarding the toolbar.
If I test the same code, and the same combination of preferences on debug mode (adding ?debug=1 to the URL), the resulting log is always the same, but different from the above:
----
$.isEmpty(mw.user.options.values)= true
load.php:3usebetatoolbar=null;showtoolbar=null
load.php:8using "user.options" > $.isEmpty(mw.user.options.values)= true
load.php:9using "user.options" > usebetatoolbar=null;showtoolbar=null
----

I don't know if I should report in a separated bug the fact that
mw.loader.using('user.options', function () {/* code */})"
doesn't guarantee that mw.user.options.values will be filled with the user's preference.
But this is certainly also a bug, because currently the /* code */ can't use mw.user.options.get() at all.

[1] https://pt.wikipedia.org/?oldid=29092806
[2] https://pt.wikipedia.org/wiki/Special:Preferences?uselang=en#mw-prefsection-editing