Popupos and MobileFrontend CI builds the dist files from the Javascript sources, and compares those against the dist files in the patch and rejects it if they aren't the same. This is a major barrier to entry and source of frustration for new contributors - it requires significant extra environment setup compared to what's needed for writing and testing the patch, and it's very erratic - sometimes you build the distfiles using the exact same npm and library versions and parent commit, and it still doesn't match.
At a minimum a clearer error message would be nice (right now it says Try running npm run build again or removing the node_modules folder and running npm install with the correct node version., even though it just asserted a few lines above that the node version matches), but maybe a better approach would be to export the dist files as CI artifacts, so instead of struggling with tracking down minuscule differences in their local setup, contributors could just copy the correct dist files from Jenkins into the patch.