Why does transitioning from width: 100% cause the transition to jump?

Question

I made a JSFiddle to reproduce this issue.

I'm trying to make a grid element grow on hover, but it causes this weird issue where it goes underneath another grid element and then jumps the way I expect it to.

Why does this happen? Is there a way to solve?

.container {
  height: 100vh;
  width: 100vw;
  display: grid;
  grid-template: 1fr / 1fr 1fr;
  margin: 1em;
  grid-gap: 1em;
}

.box {
  height: 100%;
  width: 100%;
  transition: width 0.5s;
}

.one {
  background: pink;
}

.two {
  background: red;
}

.box:hover {
  width: 60vw;
}

<div class="container">
  <div class="box one"></div>
  <div class="box two"></div>
</div>

Answer 1

I wrote a detailed article about this effect and I invite you to read it to learn how to achieve this effect using CSS Grid: https://css-tricks.com/zooming-images-in -a-grid-layout/

.container {
  height: calc(100vh - 2em);
  display: grid;
  grid-template-columns: auto auto;
  margin: 1em;
  gap: 1em;
}
.box {
  width: 0;
  min-width: 100%;
  transition: width 0.5s;
}
.box:hover {
  width: 40vw; /* read the article to understand the math behind setting this value */
}

.one {background: pink;}
.two {background: red;}

body {
  margin: 0;
}

<div class="container">
  <div class="box one"></div>
  <div class="box two"></div>
</div>

Answer 2

You can combine Flexbox with flex Shorthand properties :

.container {
  display: flex;
  gap: 1em;
  margin: 1em;
}

.box {
  flex: 1; /* This makes boxes take equal space by default */
  transition: 0.5s;
}

.box:hover {
  flex: 2; /* A hovered box expands twice as fast as a non-hovered */
}

try it:

.container {
  display: flex;
  gap: 1em;
  margin: 1em;
}

.box {
  flex: 1;
  transition: 0.5s;
}

.box:hover {
  flex: 2;
}


/* Demo only */

body {
  margin: 0;
}

.container {
  height: 100vh;
}

.box {
  height: 100%;
}

.one {
  background: pink;
}

.two {
  background: red;
}

<div class="container">
  <div class="box one"></div>
  <div class="box two"></div>
</div>