使用vh单位和调整浏览器窗口大小后CSS3动画后的最终位置

Question

I have an image that is absolutely positioned using vh units. I want to animate this positioning use CSS. When doing so, however, the relative nature of vh units seems to be lost. To illustrate, look at the following two examples. In both of them, drag the bottom of your browser up and down to change its height.

No animation

The positioning adjusts correctly in relation to the screen height.

http://codepen.io/maxedison/pen/jPOQPW

 #mountain { position: absolute; left: 50%; top: 55vh; opacity: 1; } img { width: 180vh; margin-left: -50%; } 

 <div id="screen1"> <div id="mountain"> <img src="http://upload.wikimedia.org/wikipedia/commons/2/20/Red_Slate_Mountain_1.jpg"> </div> </div> 

Animation
The positioning does NOT adjust at all. It's like the vh unites have turned into static px , maintaining the same distance from the top of the window regardless of screen height.

http://codepen.io/maxedison/pen/QbWJbj

 #mountain { position: absolute; left: 50%; top: 100vh; opacity: 0; -webkit-animation: lincoln_page_load 2s ease forwards; animation: lincoln_page_load 2s ease forwards; } img { width: 180vh; margin-left: -50%; } @-webkit-keyframes lincoln_page_load { to { opacity: 1; top: 55vh } } @keyframes lincoln_page_load { to { opacity: 1; top: 55vh } } 

 <div id="screen1"> <div id="mountain"> <img src="http://upload.wikimedia.org/wikipedia/commons/2/20/Red_Slate_Mountain_1.jpg"> </div> </div> 

Any ideas on how to correct this? I know I can resort to JavaScript to make this work :)

Answer 1

This is only a problem when the animation is paused with forwards and the animation is still active:

• Place the top: 55vh in #mountain so that when the animation ends it has this value and remove the opacity: 0

• Remove forwards so that the animation is completed

• Add the opacity: 0 and top: 100vh to from in the keyframes so that these values are present when the page loads

This has the added benefit of showing the image if the browser does not support the animation property.

Codepen Example with SASS (Auto-prefixer is turned on)

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Using a transform for animation

Here is another example using a transform — translate ( info link ) — which seems to provide a slightly smoother animation.

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Working Example — vanilla CSS

 #mountain { position: absolute; left: 50%; top: 55vh; -webkit-animation: lincoln_page_load 2s ease; animation: lincoln_page_load 2s ease; } img { width: 180vh; margin-left: -50%; } @-webkit-keyframes lincoln_page_load { from { top: 100vh; opacity: 0; } to { opacity: 1; top: 55vh } } @keyframes lincoln_page_load { from { top: 100vh; opacity: 0; } to { opacity: 1; top: 55vh } } 

 <div id="screen1"> <div id="mountain"> <img src="http://upload.wikimedia.org/wikipedia/commons/2/20/Red_Slate_Mountain_1.jpg"> </div> </div>