expand flex children to full width on hover

Question

in the example below is it possible to expand flex children to full width on hover with css only or I should do this with javascript? Note: It's important to use flexbox in this scenario. and if its not possible to create it with css can you please suggest a javascript functionality for this problem?

 html {
    box-sizing: border-box;
    background: #ffc600;
    font-family: 'sans-serif';
    font-size: 20px;
    font-weight: 200;
}

body {
    margin: 0;
}

*, *:before, *:after {
    box-sizing: inherit;
}

.panels {
    min-height: 100vh;
    overflow: hidden;
    display: flex;
    position: relative;
    
}

.panel {
    background: #6B0F9C;
    box-shadow: inset 0 0 0 5px rgba(255,255,255,0.1);
    color: white;
    text-align: center;
    align-items: center;
    transition:
        transform 0.2s,
        flex 0.7s cubic-bezier(0.61,-0.19, 0.7,-0.11),
        background 0.2s;
    font-size: 20px;
    background-size: cover;
    background-position: center;
    flex: 1;
    display: flex;
    flex-direction: column;
    justify-content: center;
}
.panel a{
    color:#fff;
    font-weight: bold;
    font-size: 20px;
    text-decoration: none;
    text-shadow: 1px 2px rgba(0,0,0,.7);
}
.panel:hover{
flex:2
}
    .panel1 { background-image:url(https://source.unsplash.com/gYl-UtwNg_I/1500x1500); }
    .panel2 { background-image:url(https://source.unsplash.com/rFKUFzjPYiQ/1500x1500); }
    .panel3 { background-image:url(https://images.unsplash.com/photo-1465188162913-8fb5709d6d57?ixlib=rb-0.3.5&q=80&fm=jpg&crop=faces&cs=tinysrgb&w=1500&h=1500&fit=crop&s=967e8a713a4e395260793fc8c802901d); }
    .panel4 { background-image:url(https://source.unsplash.com/ITjiVXcwVng/1500x1500); }
    .panel5 { background-image:url(https://source.unsplash.com/3MNzGlQM7qs/1500x1500); }

<div class="panels">
    <div class="panel panel1"><a href="">link1</a></div>
    <div class="panel panel2"><a href="">link2</a></div>
    <div class="panel panel3"><a href="">link3</a></div>
    <div class="panel panel4"><a href="">link4</a></div>
    <div class="panel panel5"><a href="">link5</a></div>
</div>

Answer 1

add min-width:0;overflow:hidden; to all the panel then on hover use flex-basis:100%

html {
  box-sizing: border-box;
  background: #ffc600;
  font-family: 'sans-serif';
  font-size: 20px;
  font-weight: 200;
}

body {
  margin: 0;
}

*,
*:before,
*:after {
  box-sizing: inherit;
}

.panels {
  min-height: 100vh;
  overflow: hidden;
  display: flex;
  position: relative;
}

.panel {
  background: #6B0F9C;
  box-shadow: inset 0 0 0 5px rgba(255, 255, 255, 0.1);
  color: white;
  text-align: center;
  align-items: center;
  transition: transform 0.2s, flex 0.7s cubic-bezier(0.61, -0.19, 0.7, -0.11), background 0.2s;
  font-size: 20px;
  background-size: cover;
  background-position: center;
  flex: 1;
  min-width:0;
  overflow:hidden;
  display: flex;
  flex-direction: column;
  justify-content: center;
}

.panel a {
  color: #fff;
  font-weight: bold;
  font-size: 20px;
  text-decoration: none;
  text-shadow: 1px 2px rgba(0, 0, 0, .7);
}

.panel:hover {
  flex-basis: 100%;
}

.panel1 {
  background-image: url(https://source.unsplash.com/gYl-UtwNg_I/1500x1500);
}

.panel2 {
  background-image: url(https://source.unsplash.com/rFKUFzjPYiQ/1500x1500);
}

.panel3 {
  background-image: url(https://images.unsplash.com/photo-1465188162913-8fb5709d6d57?ixlib=rb-0.3.5&q=80&fm=jpg&crop=faces&cs=tinysrgb&w=1500&h=1500&fit=crop&s=967e8a713a4e395260793fc8c802901d);
}

.panel4 {
  background-image: url(https://source.unsplash.com/ITjiVXcwVng/1500x1500);
}

.panel5 {
  background-image: url(https://source.unsplash.com/3MNzGlQM7qs/1500x1500);
}

<div class="panels">
  <div class="panel panel1"><a href="">link1</a></div>
  <div class="panel panel2"><a href="">link2</a></div>
  <div class="panel panel3"><a href="">link3</a></div>
  <div class="panel panel4"><a href="">link4</a></div>
  <div class="panel panel5"><a href="">link5</a></div>
</div>