Why does the window move when the user makes a selection?

[adokhugi]

In our JavaScript Vue code we have a template that looks approximately like this:

<template>
  <NiceDialog
    ref="selectMenuBody"
    closable
    modal
    v-model:visible="visible"
    :header="title"
    style="min-width: 35rem; width: auto"
    @keydown.escape.exact.stop.prevent="doHide"
    @keydown.space.stop.prevent=""
    @hide="doHide"
    @dragend="dragEnd"
    @mousedown="dragging = true"
  >
    <div class="ml-1">Charset:</div>
    <select v-model="selected" style="width: calc(100% - 0.4rem); margin: 0.2rem; height: 1.75rem">
      <option v-for="o in listItems" :key="o">{{ o }}</option>
    </select>
    <table>
      <tbody>
        <tr v-for="m in 8" :key="m - 1">
          <td v-for="n in 16" :key="(m - 1) * 16 + n - 1">
            <button
              @click.stop="_clickButton(charIfPrintable(m, n))"
              style="width: 3.3rem; height: 2.2rem"
              onclick="this.blur();"
            >
              {{ charIfPrintable(m, n) }}
            </button>
          </td>
        </tr>
      </tbody>
    </table>
  </NiceDialog>
</template>

What bothers us is that when we move the window and then make a selection in the select box, the position of the window is modified. It's not like it jumps back to a default position but the position is simply changed a bit. We would like to make the position static. How can this be done?