Magento Grunt child theme compilation fails to update merged style files - magento

Question

I've set up the Grunt workflow as described here:
https://devdocs.magento.com/guides/v2.3/frontend-dev-guide/css-topics/css_debug.html
My local-themes.js looks like this:
➜ m2 git:(master) ✗ cat dev/tools/grunt/configs/local-themes.js
'use strict';
module.exports = {
ac_default: {
area: 'frontend',
name: 'Ac/Default',
locale: 'en_US',
files: [
'css/source/_extend',
],
dsl: 'less'
},
ac_retail: {
area: 'frontend',
name: 'Ac/Retail',
locale: 'en_US',
files: [
'css/source/_extend',
],
dsl: 'less'
},
ac_wholesale: {
area: 'frontend',
name: 'Ac/Wholesale',
locale: 'en_US',
files: [
'css/source/_extend',
],
dsl: 'less'
}
};
I grunt watch successfully triggers when a less file in my child theme gets changed:
➜ m2 git:(master) ✗ grunt watch:ac_retail -v
Initializing
Command-line options: --verbose, --gruntfile=/data/src/m2/Gruntfile.js
Reading "Gruntfile.js" Gruntfile...OK
Registering Gruntfile tasks.
Initializing config...OK
Loading "Gruntfile.js" tasks...OK
+ black-list-generator, clean-black-list, default, deploy, documentation, legacy-build, mage-minify, prod, refresh, spec, static
Running tasks: watch:ac_retail
Loading "grunt-contrib-watch" plugin
Registering "/data/src/m2/node_modules/grunt-contrib-watch/tasks" tasks.
Loading "watch.js" tasks...OK
+ watch
Running "watch:ac_retail" (watch) task
Waiting...
Verifying property watch exists in config...OK
Verifying property watch.ac_retail.files exists in config...OK
Live reload server started on *:35729
Watching pub/static/frontend/Ac/Retail/en_US/css/source/_extend.less for changes.
>> File "pub/static/frontend/Ac/Retail/en_US/css/source/_extend.less" changed.
Initializing
Command-line options: --verbose, --gruntfile=/data/src/m2/Gruntfile.js
Reading "Gruntfile.js" Gruntfile...OK
Registering Gruntfile tasks.
Initializing config...OK
Loading "Gruntfile.js" tasks...OK
+ black-list-generator, clean-black-list, default, deploy, documentation, legacy-build, mage-minify, prod, refresh, spec, static
Running tasks: less:ac_retail
Loading "grunt-contrib-less" plugin
Registering "/data/src/m2/node_modules/grunt-contrib-less/tasks" tasks.
Loading "less.js" tasks...OK
+ less
Running "less:ac_retail" (less) task
Verifying property less.ac_retail exists in config...OK
Files: pub/static/frontend/Ac/Retail/en_US/css/source/_extend.less -> pub/static/frontend/Ac/Retail/en_US/css/source/_extend.css
Options: banner="", sourceMap, strictImports=false, sourceMapRootpath="/", dumpLineNumbers=false, ieCompat=false
Reading pub/static/frontend/Ac/Retail/en_US/css/source/_extend.less...OK
Writing pub/static/frontend/Ac/Retail/en_US/css/source/_extend.css.map...OK
File pub/static/frontend/Ac/Retail/en_US/css/source/_extend.css.map created.
Writing pub/static/frontend/Ac/Retail/en_US/css/source/_extend.css...OK
File pub/static/frontend/Ac/Retail/en_US/css/source/_extend.css created
>> 1 stylesheet created.
>> 1 sourcemap created.
Done.
Execution Time (2019-01-16 16:13:35 UTC-8)
loading tasks 42ms ▇▇▇▇▇▇▇▇▇▇▇▇▇ 39%
loading grunt-contrib-less 34ms ▇▇▇▇▇▇▇▇▇▇ 32%
less:ac_retail 31ms ▇▇▇▇▇▇▇▇▇ 29%
Total 107ms
Live reloading pub/static/frontend/Ac/Retail/en_US/css/source/_extend.less...
Completed in 0.729s at Wed Jan 16 2019 16:13:35 GMT-0800 (Pacific Standard Time) - Waiting...
Unfortunately the styles-m.css or styles-l.css files are not updated. Therefore when I refresh the page the changes made in the less files are not reflected on the page.
I posted a ticket in the Magento bug tracker but they have not been particularly helpful.
https://github.com/magento/magento2/issues/20356
My question is. Does anyone here uses Grunt to compile less for a setup with multiple child themes?
In my case the intended theme inheritance looks like this:
ac_retail inherits from ac_default inherits from luma
ac_wholesale inherits from ac_default inherits from luma
From what I can tell the Grunt workflow is the only workflow that is actually suitable for theme development. Server and client side compilation are way too slow to use for development.

Answer 1

Finally figured it out. The documentation has errors. You need to specify the parent themes style files, even if your child theme has it's own root source files, if you want to re-compile styles-m.css and styles-l.css, when the child themes less files change. This local-themes.js ended up working for me:
>>'use strict';
>>module.exports = {
ac_default: {
area: 'frontend',
name: 'Ac/Default',
locale: 'en_US',
files: [
'css/styles-m',
'css/styles-l',
],
dsl: 'less'
},
ac_retail: {
area: 'frontend',
name: 'Ac/Retail',
locale: 'en_US',
files: [
'css/styles-m',
'css/styles-l',
],
dsl: 'less'
},
ac_wholesale: {
area: 'frontend',
name: 'Ac/Wholesale',
locale: 'en_US',
files: [
'css/styles-m',
'css/styles-l',
],
dsl: 'less'
}
};

Answer 2

adding
'css/styles-m',
'css/styles-l',
fixed it for me too. Thanks for this