The mobile.messageBox module has both a stylesheet, and a JS file, HTML template, and a module dependency.

The module is loaded in three ways, it seems this is probably not done so intentionally but happens to work right now (or is broken already?)

1. SkinMinerva::getSkinStyles - Uses addModuleStyles. Loads in the blocking stylesheet without JS. Ignores the JS, template and dependency.
2. onBeforePageDisplay hook - Uses addModuleStyles. Loads in the blocking stylesheet without JS. Ignores the JS, template and dependency.
3. Indirectly as dependency of mobile.nearby or mobile.editor.common - Depending on whether the page loaded the module as styles-only already, this may or may not actually deliver the HTML and JS.

A module can either be a styles-module intended for loading in a blocking stylesheet, or it is a general module that has (or may also have) JS content and supports dependencies, messages, and templates. You cannot load a general module's stylesheet through addModuleStyles() and ignore the rest of the module (which cannot be loaded with CSS). For example, right now this module is loaded with addModuleStyles in two places, which suggests that the styles of this module apply to server-generated HTML (not related to dynamically created elements by JavaScript). But they're also in the same module as a JavaScript file, which suggests this needs those styles somehow as well.

When the module is dependent on this, does that mean it depends on the styles, or the JS as well? Should the third case load the entire module again (thus loading the styles for a second time) or was the first time "really" done.

Soon, the latter will be assumed per T92459, which fixes many bugs. However that will break this use case.

Perhaps the script is unused and the module is treated as styles-only? Or perhaps it needs to be split up, or have the JS portions moved to a different module.

Change 354222 had a related patch set uploaded (by Phuedx; owner: Phuedx):
[mediawiki/extensions/MobileFrontend@master] Hygiene: Break out mobile.messageBox styles module

https://gerrit.wikimedia.org/r/354222

Change 354225 had a related patch set uploaded (by Phuedx; owner: Phuedx):
[mediawiki/extensions/MobileFrontend@master] rl: Break out mobile.mainMenu style module

https://gerrit.wikimedia.org/r/354225

Thanks for your review @bmansurov!

Change 354222 merged by jenkins-bot:
[mediawiki/extensions/MobileFrontend@master] rl: Break out mobile.messageBox styles module

https://gerrit.wikimedia.org/r/354222

Change 354225 merged by jenkins-bot:
[mediawiki/extensions/MobileFrontend@master] rl: Break out mobile.mainMenu style module

https://gerrit.wikimedia.org/r/354225

Sounds like this is related to the https://gerrit.wikimedia.org/r/361455 (T125174)

Change 363651 had a related patch set uploaded (by Jdlrobson; owner: Jdlrobson):
[mediawiki/extensions/MobileFrontend@master] Only message box styles should be loaded on editor

https://gerrit.wikimedia.org/r/363651

^ @Krinkle

Change 363651 merged by jenkins-bot:
[mediawiki/extensions/MobileFrontend@master] Only message box styles should be loaded on editor

https://gerrit.wikimedia.org/r/363651

Change 363741 had a related patch set uploaded (by Jdlrobson; owner: Jdlrobson):
[mediawiki/extensions/MobileFrontend@wmf/1.30.0-wmf.7] Only message box styles should be loaded on editor

https://gerrit.wikimedia.org/r/363741

Change 363741 merged by jenkins-bot:
[mediawiki/extensions/MobileFrontend@wmf/1.30.0-wmf.7] Only message box styles should be loaded on editor

https://gerrit.wikimedia.org/r/363741