found the culprit in building see-gateway
the file in the source tree is a link
ls -lrt src/main/resources/org/jenkinsci/plugins/ssegateway/sse/EventSource.js lrwxr-xr-x 1 olamy staff 77 13 Feb 19:45 src/main/resources/org/jenkinsci/plugins/ssegateway/sse/EventSource.js -> ../../../../../../../../node_modules/event-source-polyfill/eventsource.min.jsdifferent result
there is a difference when copying resources in 2.7 and >=3.0.0
maven-resources-plugin:2.7:resources
ls -lrt target/classes/org/jenkinsci/plugins/ssegateway/sse/
rw-r-r- 1 olamy staff 5226 13 Feb 21:04 target/classes/org/jenkinsci/plugins/ssegateway/sse/EventSource.js
maven-resources-plugin:3.1.0:resources
ls -lrt target/classes/org/jenkinsci/plugins/ssegateway/sse/total 0lrwxr-xr-x 1 olamy staff 77 13 Feb 21:01 EventSource.js -> ../../../../../../../../node_modules/event-source-polyfill/eventsource.min.js
with 2.7 version of the plugin the file is copied as a file and not as a link as done by version >=3.0.0
Seeing same issue here - Blue Ocean shows a white screen on Ms Edge
blueocean 1.19.0
sse-gateway 1.20