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https://www.w3schools.com/howto/tryit.asp?filename=tryhow_js_fullscreen_search

<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
<style>
body {
  font-family: Arial;
}

* {
  box-sizing: border-box;
}

.openBtn {
  background: #f1f1f1;
  border: none;
  padding: 10px 15px;
  font-size: 20px;
  cursor: pointer;
}

.openBtn:hover {
  background: #bbb;
}

.overlay {
  height: 100%;
  width: 100%;
  display: none;
  position: fixed;
  z-index: 1;
  top: 0;
  left: 0;
  background-color: rgb(0,0,0);
  background-color: rgba(0,0,0, 0.9);
}

.overlay-content {
  position: relative;
  top: 46%;
  width: 80%;
  text-align: center;
  margin-top: 30px;
  margin: auto;
}

.overlay .closebtn {
  position: absolute;
  top: 20px;
  right: 45px;
  font-size: 60px;
  cursor: pointer;
  color: white;
}

.overlay .closebtn:hover {
  color: #ccc;
}

.overlay input[type=text] {
  padding: 15px;
  font-size: 17px;
  border: none;
  float: left;
  width: 80%;
  background: white;
}

.overlay input[type=text]:hover {
  background: #f1f1f1;
}

.overlay button {
  float: left;
  width: 20%;
  padding: 15px;
  background: #ddd;
  font-size: 17px;
  border: none;
  cursor: pointer;
}

.overlay button:hover {
  background: #bbb;
}
</style>
</head>
<body>

<h2>Fullscreen Search</h2>
<p>Click on the button to open the search box.</p>
<button class="openBtn" onclick="openSearch()">Open Search Box</button>

function closeSearch() {
  document.getElementById("myOverlay").style.display = "none";
}
</script>
     
</body>
</html>

https://www.w3schools.com/howto/tryit.asp?filename=tryhow_js_fullscreen_search

<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
<style>
body {
font-family: Arial;
}

* {
box-sizing: border-box;
}

.openBtn {
background: #f1f1f1;
border: none;
padding: 10px 15px;
font-size: 20px;
cursor: pointer;
}

.openBtn:hover {
background: #bbb;
}

.overlay {
height: 100%;
width: 100%;
display: none;
position: fixed;
z-index: 1;
top: 0;
left: 0;
background-color: rgb(0,0,0);
background-color: rgba(0,0,0, 0.9);
}

.overlay-content {
position: relative;
top: 46%;
width: 80%;
text-align: center;
margin-top: 30px;
margin: auto;
}

.overlay .closebtn {
position: absolute;
top: 20px;
right: 45px;
font-size: 60px;
cursor: pointer;
color: white;
}

.overlay .closebtn:hover {
color: #ccc;
}

.overlay input[type=text] {
padding: 15px;
font-size: 17px;
border: none;
float: left;
width: 80%;
background: white;
}

.overlay input[type=text]:hover {
background: #f1f1f1;
}

.overlay button {
float: left;
width: 20%;
padding: 15px;
background: #ddd;
font-size: 17px;
border: none;
cursor: pointer;
}

.overlay button:hover {
background: #bbb;
}
</style>
</head>
<body>

<div id="myOverlay" class="overlay">
<span class="closebtn" onclick="closeSearch()" title="Close Overlay">�</span>
<div class="overlay-content">
<form action="/action_page.php">
<input type="text" placeholder="Search.." name="search">
<button type="submit"><i class="fa fa-search"></i></button>
</form>
</div>
</div>

<h2>Fullscreen Search</h2>
<p>Click on the button to open the search box.</p>
<button class="openBtn" onclick="openSearch()">Open Search Box</button>

<script>
function openSearch() {
document.getElementById("myOverlay").style.display = "block";
}

function closeSearch() {
document.getElementById("myOverlay").style.display = "none";
}
</script>

</body>
</html>