Version

3.0.0-rc.3

Reproduction link

https://jsfiddle.net/fyAgent/cmyjdsvf/

Steps to reproduce

// edit vue.config.js;
module.exports = {
assetsDir: undefined,
baseUrl: undefined,
outputDir: undefined,
runtimeCompiler: undefined,
productionSourceMap: undefined,
parallel: undefined,
css: undefined,
devServer:{
port:9000,
host:"localhost"
}
}
run vue ui
run serve

What is expected?

it will work correct when i change the configuration of devServer

What is actually happening?

when use vue ui to start the server,it will still open at port 8080