@Iniquity I believe there was an OOUI release last week, which might be related, and wouldn't be surprising as I don't think we actively test on this skin.

It works fine in Vector, the reason it's a problem in MonoBook is the skinStyle rule here is no longer applying:
https://github.com/wikimedia/mediawiki-skins-MonoBook/blob/2f2b16b3956c18b604ea0202b8da301a5607b3ca/resources/skinStyles/ooui.less#L3

Thanks for creating a separate task :)

In T290026#7320560, @Xaosflux wrote:

Temporary CSS hack added to enwiki, may be overly broad

.oo-ui-labelElement-label{
 font-size: small;
}

The correct hack would be to add the code from here instead: https://github.com/wikimedia/mediawiki-skins-MonoBook/blob/2f2b16b3956c18b604ea0202b8da301a5607b3ca/resources/skinStyles/ooui.less

Your version actually slightly changes the font-size of all interfaces using OOUI (e.g. the form on Special:Contributions, and various dialogs in visual editor), and does not correct the font size in other dialogs and dropdowns (e.g. the calendar dropdowns on the date fields on Special:Contributions).

This was probably caused by rMW1367e356e76e: resourceloader: Remove isFileModule() overhead from web requests and it looks like @Krinkle is already working on a fix.

Trying the more targeted hack on enwiki now (https://en.wikipedia.org/w/index.php?title=MediaWiki:Monobook.css&action=history) - waiting for cache updates

(seems to be working)

Local wiki with Vector and MonoBook installed:

# SkinStyleModules,
# as injected through ServiceWiring.php
  'vector' =>  array
      ..
      '+oojs-ui-core.styles' => 'skinStyles/ooui.less'
      'localBasePath' => '/var/www/mediawiki/skins/Vector'
      'remoteSkinPath' => 'Vector'
  'monobook' => array
     ..
      '+oojs-ui-core.styles' => 'resources/skinStyles/ooui.less'
      'localBasePath' => '/var/www/mediawiki/skins/MonoBook'
      'remoteSkinPath' => 'MonoBook'

# initial FileModule#skinStyles
{
    "default": [
        "resources/lib/ooui/oojs-ui-core-wikimediaui.css"
    ],
    "monobook": [
        "resources/lib/ooui/oojs-ui-core-apex.css"
    ]
}

# FileModule instance for 'oojs-ui-core.styles'
# receiving SkinStyles via setSkinStylesOverride
extra skinStyles from vector
{
    ..
    "+oojs-ui-core.styles": "skinStyles/ooui.less",
    "localBasePath": "/var/www/mediawiki/skins/Vector",
    "remoteSkinPath": "Vector"
}
extra skinStyles from monobook
{
    ..
    "+oojs-ui-core.styles": "resources/skinStyles/ooui.less",
    "localBasePath": "/var/www/mediawiki/skins/MonoBook",
    "remoteSkinPath": "MonoBook"
}

# FileModule instance for 'oojs-ui-core.styles'
# merged and applied SkinStyles by end of setSkinStylesOverride
  'default'  array
      0 => 'resources/lib/ooui/oojs-ui-core-wikimediaui.css'
  'monobook'  array
      0 => 'resources/lib/ooui/oojs-ui-core-apex.css'
  'vector' array
      0 => 'resources/lib/ooui/oojs-ui-core-wikimediaui.css'
      1 =>
        object(ResourceLoaderFilePath)[277]
          localBasePath => '/var/www/mediawiki/skins/Vector'
          remoteBasePath => '/mediawiki/skins/Vector'
          path => 'skinStyles/ooui.less'

It seems the MonoBook one isn't getting merged for some reason.

Change 715786 had a related patch set uploaded (by Krinkle; author: Krinkle):

[mediawiki/core@master] resourceloader: Fix prepending of OOUI theme skinStyles

https://gerrit.wikimedia.org/r/715786

Change 715787 had a related patch set uploaded (by Krinkle; author: Krinkle):

[mediawiki/core@master] resourceloader: Document and simplify OOUIFileModule::extendSkinSpecific

https://gerrit.wikimedia.org/r/715787

Change 715810 had a related patch set uploaded (by Krinkle; author: Krinkle):

[mediawiki/core@wmf/1.37.0-wmf.21] resourceloader: Fix prepending of OOUI theme skinStyles

https://gerrit.wikimedia.org/r/715810

Change 715786 merged by jenkins-bot:

[mediawiki/core@master] resourceloader: Fix prepending of OOUI theme skinStyles

https://gerrit.wikimedia.org/r/715786

Change 715787 merged by jenkins-bot:

[mediawiki/core@master] resourceloader: Document and simplify OOUIFileModule::extendSkinSpecific

https://gerrit.wikimedia.org/r/715787

Change 715818 had a related patch set uploaded (by Krinkle; author: Krinkle):

[mediawiki/core@wmf/1.37.0-wmf.20] resourceloader: Fix prepending of OOUI theme skinStyles

https://gerrit.wikimedia.org/r/715818

Change 715810 merged by jenkins-bot:

[mediawiki/core@wmf/1.37.0-wmf.21] resourceloader: Fix prepending of OOUI theme skinStyles

https://gerrit.wikimedia.org/r/715810

Change 715818 merged by jenkins-bot:

[mediawiki/core@wmf/1.37.0-wmf.20] resourceloader: Fix prepending of OOUI theme skinStyles

https://gerrit.wikimedia.org/r/715818