Prevent CSS transform affects children

I'm trying to apply CSS transform property to two elements(Parent and child) But the problem is when i adding transform to parent then it affects child positioning.

An Example:

$('.dropdownToggler').click(function() {
  $('.dropdown').toggleClass('-isOpen')
});

$('.test').click(function() {
  $('.topbar').toggleClass('transform');
});

* {
  box-sizing: border-box;
}

.topbar {
  position: fixed;
  height: 45px;
  background: #333;
  top: 0;
  right: 0;
  left: 0;
  padding: 12px;
  color: #fff;
  /* remove comment below */
  /*transform: translateY(0);*/
}

.topbar.transform {
  transform: translateY(0);
}

.dropdown {
  background: #ddd;
  height: 100%;
  position: fixed;
  top: 45px;
  right: 0;
  left: 0;
  color: #333;
  /* main styles */
  transform: translateY(100%);
  transition: transform 300ms ease-in-out;
  will-change: transform;
}

.dropdown.-isOpen {
  transform: translateY(0%);
}

.test {
  position: fixed;
  bottom: 20px;
  right: 20px;
  z-index: 1;
}

<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>

<body>
  <div class="topbar">
    <script src="https://code.jquery.com/jquery-1.7.2.js"></script>
    <button class="dropdownToggler">Open Menu</button>
    <button class="test">toggle `transform` to parent</button>
    <div class="dropdown">Notifications</div>
  </div>

</body>

</html>

When you add transform: translateY(0); to the topbar (Parent) then notifications panel(Child) positioning based on parent. I'm trying to prevent this behavior.

For smoother animations, I've used transform, of course, we can use CSS top. Any suggestion? Thank you for devoting your time.

Solution

You could set the height of .dropdown in viewport units (100vh - the height of the topbar)

Example:

$('.dropdownToggler').click(function() {
  $('.dropdown').toggleClass('-isOpen')
});

$('.test').click(function() {
  $('.topbar').toggleClass('transform');
});

* {
  box-sizing: border-box;
}

.topbar {
  position: fixed;
  height: 45px;
  background: #333;
  top: 0;
  right: 0;
  left: 0;
  padding: 12px;
  color: #fff;
  /* remove comment below */
  /*transform: translateY(0);*/
}

.topbar.transform {
  transform: translateY(0);
}

.dropdown {
  background: #ddd;
  height: calc(100vh - 45px);
  position: fixed;
  top: 45px;
  right: 0;
  left: 0;
  color: #333;
  /* main styles */
  transform: translateY(100%);
  transition: transform 300ms ease-in-out;
  will-change: transform;
}

.dropdown.-isOpen {
  transform: translateY(0%);
}

.test {
  position: fixed;
  bottom: 20px;
  right: 20px;
  z-index: 1;
}

<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>

<body>
  <div class="topbar">
    <script src="https://code.jquery.com/jquery-1.7.2.js"></script>
    <button class="dropdownToggler">Open Menu</button>
    <button class="test">toggle `transform` to parent</button>
    <div class="dropdown">Notifications</div>
  </div>

</body>

</html>