'devise toolbar' (a.k.a. mobile mode) turned on, it makes scrollbar fat

If I open the Chrome dev tool with 'devise toolbar' (a.k.a. mobile mode) turned on, the scrollbar will become thicker. Of course, if refresh or change window size, the scrollbar will come back to normal. But i'm curious. why is this happening?

Example

css

.outer{
    white-space: nowrap;
    overflow-x: auto;
}

.item{
    display: inline-block;
    width: 200px;
    height: 200px;
    background-color: #333;
}

html

<div class="outer">
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
</div>