I have a problem with understanding the event system

I have this code:

<template>

  <div class="chart"
       v-bind:style="chartStyleObject"
       v-on:mousedown.left="initHandleMousedown($event)"
       v-on:mouseup.left="initHandleMouseup()"
       v-on:mouseout="initHandleMouseup()">

    <div class="chartContent">
    </div>
    <!--   <div class="chartContent">  end   -->

  </div>
  <!--   <div class="chart">   end   -->

</template>

<script>
  import axios from 'axios';

export default{

created () {
},

data () {
  return {
    ticket: null,

    chartStyleObject: {
      width: '500px',
      widthWrapper: '1600px',
      heightWrapper: '500px',
      height: '247px',
      marginTop: '15px',
      marginRight: '0px',
      marginBottom: '0px',
      marginLeft: '15px',
    },

    XCoord: null,
    YCoord: null,

  }
},

methods: {

  initHandleMousedown(event) {
    this.startMousedownXCoord = event.clientX;
    this.startMousedownYCoord = event.clientY;
    this.XCoord = event.clientX;
    this.YCoord = event.clientY;

    console.log('XCoord', this.XCoord);
    console.log('YCoord', this.YCoord);

    window.addEventListener('mousemove', this.initHandleMouseMove);
  },

  initHandleMouseMove(event) {

      this.XCoord = event.clientX;
      this.YCoord = event.clientY;

      console.log('XCoord', this.XCoord);
      console.log('YCoord', this.YCoord);

  },

  initHandleMouseup() {
    window.removeEventListener('mousemove', this.initHandleMouseMove);
  },

  },

}

</script>

<style scoped>

.chart{
  position: relative;
  border-radius: 10px;
  padding: 27px 10px 10px 10px;
  background-color: #45788b;
  box-sizing: border-box;
  cursor: move;
}
.chart .chartContent{
  position: relative;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
  margin: 0 0 0 0;
  background-color: #2f2c8b;
}



</style>

HTML design consists of 2 blocks:
(parent and child)
The event is tied to the parent tag `<div class =" chart ">`
Also, the parent block has padding on all 4 sides:

If you click on the parent block and drive with the mouse (holding the button pressed) without affecting the padding space, the mousemove event will fire without problems. But as soon as the mouse cursor touches the padding territory, the event ceases to function.
If you click on the padding, the event also works correctly - but it stops working if I move the mouse cursor over the block space outside the paddings (internal space)
Question:
Why is this happening - and is this behavior normal for js + nuxt.js?

from I have a problem with understanding the event system