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A Viewport that Works


Problem

CSS provides easy ways to measure content on screen according to the viewport, but some browsers and some devices don't quite behave the way you'd expect.

Example (Demo)

Keyboard closed

Notice the "end of viewport" label at the bottom.


Keyboard open

Once we toggle the Safari keyboard, the viewport calculation does not take into account the space obscured by the keyboard.


With A Viewport that Works, we can fix this instantly with very little CSS and JavaScript

Keyboard closed


Keyboard open


Installation

Quickstart

Copy index.html and delete these lines

<!-- To use this HTML file in a real project, remove these lines. -->
<link rel="stylesheet" href="css/demo.css">
<script src="js/demo.js"></script>

You'll now have a blank canvas like this:


Now just treat <div id="viewport"></div> as if it were the <body> tag.

From scratch

In an existing HTML file, you'll need the following:

  1. Import normalize and viewport-fix.css in the <head>

    <link rel="stylesheet" href="css/normalize.css">
<link rel="stylesheet" href="css/viewport-fix.css">

  2. <div id="viewport"></div> Directly under the <body> tag.

  3. Import and instantiate viewport-fix.js

    <script src="js/viewport-fix.js"></script>
<script>new VVP()</script>

Now just treat <div id="viewport"></div> as if it were the <body> tag.

How to use

Viewport Measurements

Use the CSS variables --100vvh and --100vvw in place of 100vh and 100vw respectively. I recommend passing in the native 100vh or 100vw as the fallback value. This will allow the browser to use the regular viewport height/width for the split second until the visual viewport has done its initial calculation.

Example

.sidebar {
  height: var(--100vvh, 100vh);
}

For partial measurements, use calc() with a decimal value.

Example

Instead of

.button { 
  height: 20vh;
}

...do this

.button {
  height: calc(var(--100vvh, 100vh) * 0.2);
}

Positioning

Absolute

<div id="viewport"></div> is meant to work as if it were the <body> of the document. As such, viewport-fix.css sets it as position: relative and overflow: auto.

This means as long as elements are in #viewport with no relative positioned elements in between, you can simply absolute position your elements as you would in <body>.

Example

.bottom-menu-bar {
  position: absolute;
  bottom: 0;
  left: 0;
  right: 0;
}

Fixed

Fixed positioning is a little different, so the values --offset-h and --offset-w are provided.

Example

Instead of

.sticky-bottom-menu-bar {
  position: fixed;
  left: 0;
  right: 0;
  bottom: 0;
}

...do this

.sticky-bottom-menu-bar {
  position: fixed;
  left: 0;
  right: 0;
  bottom: var(--offset-h, 0);
}

Example with offset

If a nonzero number is required, again, just use CSS calc()

Instead of

.sticky-bottom-menu-bar {
  position: fixed;
  left: 0;
  right: 0;
  bottom: 20px;
}

...do this

.sticky-bottom-menu-bar {
  position: fixed;
  left: 0;
  right: 0;
  bottom: calc(var(--offset-h, 0) + 20px);
}

How does it work?

In order to use as little JavaScript as possible, we listen to the new window.visualViewport resize() event and pull from the visualViewport height and width.

To let you keep writing CSS, these values are written into the DOM in a style tag as so:

<style id="viewport_fix_variables">
  :root {
    --100vvw: 1503px;
    --100vvh: 1082px;
    
    --offset-w: 0px;
    --offset-h: 0px;
  }
</style>

By simply using native CSS functions for variables and calculation, the CSS will automatically respect these values in place of vh and vw.