How to remove mysterious white area in a vue app?

I'm trying to build a website with vue 3 and set a background picture for it. But there is a mysterious white area that can't be removed. I'm sure it's not margin or padding because I've set all the margins and paddings to 0, and removed all ml ,pl classes. I set the blackground color of the app component to black but the area it's still white. The chrome's inspect tool says it's a html element.Please give me a hand.(I put the backgroup picture in the style part of Login.vue.)

Information from browser's inspect tool:

Inspect with pesticide:

<!-- index.html -->
<!DOCTYPE html>
<html lang="">
  <head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width,initial-scale=1.0">
    <link rel="icon" href="<%= BASE_URL %>favicon.ico">
    <title><%= htmlWebpackPlugin.options.title %></title>
  </head>
  <body>

    <div id="app"></div>
    <!-- built files will be auto injected -->
  </body>
</html>

<!-- app.vue --!>
<template>
    <div id="app">
      <div class="container">
        <router-view />
      </div>
    </div>
</template>

<script>
export default {
  name: "app"
};
</script>

<style>
#app {
  padding: 0;
  margin: 0px;
  background-color: black;
}
</style>

I put the backgroup picture in the style part of Login.vue.

<!-- Login.vue --!>
<template>
  <div class="body" id="poster">
    <el-form class="login-container">
      <el-card>
        <h4 class="login-title mb-2">Log in</h4>
        <el-form-item>
          <input type="text" class="form-control" id="username"
                 v-model="loginForm.username" placeholder="username"
          />
        </el-form-item>

        <el-form-item>
          <input type="text" class="form-control" id="password"
                 v-model="loginForm.password" placeholder="password"
          />
        </el-form-item>
        <el-form-item>
          <el-button @click="login" type="primary"
                     style="width: 100%">Submit</el-button>
        </el-form-item>
      </el-card>

    </el-form>
  </div>
</template>

<script>
import http from '../http'
export default {
  // eslint-disable-next-line vue/multi-word-component-names
  name: 'login',
  data() {
    return {
      loginForm: {
        username: '',
        password: ''
      },
      responseResult: [],
    }
  },
  methods: {
    login() {
      const data = {
        username: this.loginForm.username,
        password: this.loginForm.password
      }
      http.post('/login', data)
          .then(response => {
            console.log(data);
            if (response.data.code === 200) {
              this.$router.push('/index')
            }
          })
          .catch(e => {
            console.log(e);
          })
    }
  }
}
</script>


<style scoped>
.login-title {
  text-align: center;
}
body {
}
#poster {
  background: url("../assets/backgroud.jpg") no-repeat center;
  height: 100%;
  width: 100%;
  background-size: cover;
  position: fixed;
}
</style>

// main.js
import { createApp } from 'vue'
import App from './App.vue'
import router from "./router";
import 'bootstrap'
import 'bootstrap/dist/css/bootstrap.min.css'
import ElementPlus from 'element-plus'
import 'element-plus/dist/index.css'



const app = createApp(App)
app.use(ElementPlus)
app.use(router)
app.mount('#app')

// router.js
import { createWebHistory, createRouter } from "vue-router";

const routes = [
    {
        path: '/',
        alias: '/login',
        component: () => import('@/components/Login.vue')
    },
    {
        path: '/index',
        component: () => import('@/components/AppIndex.vue')
    }
]

const router = createRouter({
    history: createWebHistory(),
    routes
})

export default router;

Solution

The use of bootstrap class container wrapping your page content causes the max-width to be smaller than the full screen width. The documentation shows the max-width values for each breakpoint. Not only that, but bootstrap containers also have padding.

If you want to remove the empty space you should set the container width to 100% or use class name container-fluid which does the same thing, and also remove padding which you can do with class p-0

<div class="container-fluid p-0">
  <router-view />
</div>

On a side note, I noticed you're using both Element Plus and Bootstrap, which are both large CSS/component frameworks. It's very likely moving forward you'll run into a lot of frustrating conflicts between the two frameworks and their competing CSS styles. I suggest using only one of those frameworks!