Background:
Having confidence in progress made on T127268 is dependent on Logstash telemetry from violations, which would currently be too spammy due to Gadgets being loaded unconditionally and violating the targets filter by default without a way for users to fix that. (T171180).
Analysis from @TheDJ shows that the way Gadgets enqueues the enabled Gadget modules could easily filter by target, but this is currently not working as intended because the `OutputPage::mTarget` member is not set by the time Gadgets queues its modules.
Further progress on the matter is blocked on moving the setTarget() call in MobileFrontend to an earlier hook. This is expected to be uncontroversial given the target state is declarative and aside from ResourceLoader (which only consumes it upon skin rendering) nothing reads from this variable.
= Developer notes
[x] Existing code inside the onBeforePageDisplay hook setting target is removed.
[x] The following code is added to onOutputPageBeforeHTML in includes/MobileFrontend.hooks.php:
```
if ( $context->shouldDisplayMobileView() ) {
// set the mobile target
if ( !$context->isBlacklistedPage() ) {
$out->setTarget( 'mobile' );
}
}
```
= QA steps
[] When I visit a mobile page all requests for JavaScript and stylesheets append the query string parameter target=mobile
= Acceptance criteria / Sign off steps
[] There has been no increase in JS/CSS related to this change
[x] MediaWiki:Mobile.css/js is loaded on the mobile domain
[] MediaWiki:Common.css/js is not loaded on the mobile domain