I ended up killing the test for now. I got docker working locally but there's something strange going on I need to dig into. I also swapped the order of the path declarations:

   protected function getDirectories() {
     return $this->themeHandler->getThemeDirectories() + $this->moduleHandler->getModuleDirectories();
   }

Instead of

   protected function getDirectories() {
     return $this->moduleHandler->getModuleDirectories() + $this->themeHandler->getThemeDirectories();
   }

This seems to prevent the failure on the UiPatternsLibraryOverviewTest we got in the first run. I want to confirm everything works here now, and will sort out the theme switching test in a bit.

Hi @WidgetsBurritos,

I debugged the failing test in UiPatternsLibraryOverviewTest with the original order for the union of the path arrays:

   protected function getDirectories() {
     return $this->moduleHandler->getModuleDirectories() + $this->themeHandler->getThemeDirectories();
   }

I found that the issue is that the order of the displayed patterns in the overview test is different when using the module paths + theme paths version. This can be fixed by moving the 3 theme patterns (simple, with_variants and with_custom_theme_hook) to the bottom of the file in modules/ui_patterns_library/tests/fixtures/overview-page-patterns.yml

By making this change in the fixture I could run all the tests without failures.