How to update Wagtail static files, they stopped working when I used STATIC_DIRS

I'm trying to update Wagtail from version 2.8 to the latest 2.15. Also, I had to update from Django 3.0 to 3.2. However, I noticed that when I use STATICFILES_DIRS, the style of the Wagtail admin (2.15) gets distorted as if it's using the files from the old version (2.8).

PROJECT_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
BASE_DIR = os.path.dirname(PROJECT_DIR)

...

STATICFILES_FINDERS = [
   'django.contrib.staticfiles.finders.FileSystemFinder',
   'django.contrib.staticfiles.finders.AppDirectoriesFinder',
]


STATICFILES_DIRS = [os.path.join(PROJECT_DIR, 'static'),]

STATICFILES_STORAGE = 'django.contrib.staticfiles.storage.ManifestStaticFilesStorage'

STATIC_ROOT = os.path.join(BASE_DIR, 'static')
STATIC_URL = '/static/'

MEDIA_ROOT = os.path.join(BASE_DIR, 'media')
MEDIA_URL = '/media/'

Conversely, when I delete the STATICFILES_DIRS, the style on the Wagtail admin gets fixed, but the all the style on my project its gone.

I already checked and it's not the cache. I tried that several times, different browsers, etc.

Your STATICFILES_FINDERS setting tells Django that it should look for static files in the following places:

• FileSystemFinder tells it to look in whichever locations are listed in STATICFILES_DIRS;
• AppDirectoriesFinder tells it to look in the static folder of each registered app in INSTALLED_APPS.

In normal circumstances, STATICFILES_DIRS should not make a difference to Wagtail's own static files. This is because Wagtail's static files are stored within the apps that make up the Wagtail package, and will be pulled in by AppDirectoriesFinder - FileSystemFinder (and STATICFILES_DIRS) do not come into play.

The fact that you're seeing a difference suggests to me that you've previously customised Wagtail's JS / CSS by placing static files within your project's 'static' folder, in a location such as myproject/static/wagtailadmin/css/, to override the built-in files. These customisations would presumably have been made against Wagtail 2.8 and will not behave correctly against Wagtail 2.15. The solution is to remove these custom files from your project.

Try changing:

STATICFILES_DIRS = [os.path.join(PROJECT_DIR, 'static'),]

to

STATICFILES_DIRS = [os.path.join(BASE_DIR, 'static'),]