如何将绝对定位的div水平居中于其相对定位的父级？

Question

I have a speech-bubble class that has a dynamic size and is absolutely positioned. I want it to always be horizontally centered over its parent element which is relatively positioned, no matter what the speech-bubble's size is. I want it to look something like this (where the letter 'O' is the parent relatively positioned element):

Currently, it looks like this if I set the left attribute to 0:

This is an easy problem to solve if I can guarantee that the size of the bubble is fixed, but it is most definitely not. I can't solve this by setting width or left or margin-left on an element with class bubble . It needs to be more robust than that.

I would prefer a css solution if there is one. However, if the only solution is a javascript one, I will consider that as well.

This is what the code looks like. Note that the margins on smart-bubble-wrapper are not really needed, they only exist so that you can see the bubble in the code sample:

 .bubble-wrapper { position: relative; margin-top: 100px; margin-left: 100px; } .bubble { display: block; position: absolute; padding: 10px; background: #FFFFFF; -webkit-border-radius: 10px; -moz-border-radius: 10px; border-radius: 10px; border: solid 1px #bababf; box-shadow: 0px 0px 5px #6bcded; left: 0; color: #333333; } .bubble-bottom { bottom: 30px; } .bubble-middle-after { left: calc(50% - 8px); } .bubble-middle-before { left: calc(50% - 10px); } .bubble-bottom-before { bottom: -8px; } .bubble-bottom-after { bottom: -12px; border-width: 12px 12px 0; border-style: solid; border-color: #FFFFFF transparent; } .bubble-after { content: ''; position: absolute; display: block; } .bubble-before { content: ''; position: absolute; border-style: solid; border-color: #bababf; border-width: 13px 13px 0; background-color: white; display: block; box-shadow: 0px 0px 5px #6bcded; width: 0px; height: 10px; z-index: -1; transform: rotate(45deg); -moz-transform: rotate(45deg); -ms-transform: rotate(45deg); -o-transform: rotate(45deg); -webkit-transform: rotate(45deg); } 

 <div class="bubble-wrapper"> O <div class="bubble bubble-middle bubble-bottom" arrow-horizontal="middle" arrow-vertical="bottom"> <div class="bubble-before bubble-middle-before bubble-bottom-before"></div> <span class="ng-scope">How do I center this bubble</span> <div class="bubble-after bubble-middle-after bubble-bottom-after"></div> </div> </div> 

Answer 1

You can use

1. Choose a big enough length x
2. Set left to some negative value -x
3. Set right to calc(100% - x)
4. Assuming x is big enough, the absolutely positioned element will be centered at the left.
5. But its width will be 2*x . Use width: fit-content or width: max-content to fix this.
6. But now it's over-constrained. Set margin-left and margin-right to auto to keep the centering.

For example, choosing x = 100% ,

left: -100%;
right: 0;
width: fit-content; /* may need vendor prefixes */
margin: 0 auto;

 .bubble-wrapper { position: relative; margin-top: 100px; margin-left: 100px; } .bubble { display: block; position: absolute; padding: 10px; background: #FFFFFF; -webkit-border-radius: 10px; -moz-border-radius: 10px; border-radius: 10px; border: solid 1px #bababf; box-shadow: 0px 0px 5px #6bcded; color: #333333; left: -100%; right: 0; width: -webkit-fit-content; width: -moz-fit-content; width: fit-content; /* may need vendor prefixes */ margin: 0 auto; } .bubble-bottom { bottom: 30px; } .bubble-middle-after { left: calc(50% - 8px); } .bubble-middle-before { left: calc(50% - 10px); } .bubble-bottom-before { bottom: -8px; } .bubble-bottom-after { bottom: -12px; border-width: 12px 12px 0; border-style: solid; border-color: #FFFFFF transparent; } .bubble-after { content: ''; position: absolute; display: block; } .bubble-before { content: ''; position: absolute; border-style: solid; border-color: #bababf; border-width: 13px 13px 0; background-color: white; display: block; box-shadow: 0px 0px 5px #6bcded; width: 0px; height: 10px; z-index: -1; transform: rotate(45deg); -moz-transform: rotate(45deg); -ms-transform: rotate(45deg); -o-transform: rotate(45deg); -webkit-transform: rotate(45deg); } 

 <div class="bubble-wrapper"> O <div class="bubble bubble-middle bubble-bottom" arrow-horizontal="middle" arrow-vertical="bottom"> <div class="bubble-before bubble-middle-before bubble-bottom-before"></div> <span class="ng-scope">How do I center this bubble</span> <div class="bubble-after bubble-middle-after bubble-bottom-after"></div> </div> </div> 

For browsers that don't support width: fit-content , you can add an inline-block inner wrapper, which can be centered with text-align: center .

 .bubble-wrapper { position: relative; margin-top: 100px; margin-left: 100px; } .bubble { display: block; position: absolute; left: -100%; right: 0; text-align: center; margin: 0 auto; } .bubble-inner-wrapper { position: relative; display: inline-block; vertical-align: middle; padding: 10px; background: #FFFFFF; -webkit-border-radius: 10px; -moz-border-radius: 10px; border-radius: 10px; border: solid 1px #bababf; box-shadow: 0px 0px 5px #6bcded; color: #333333; } .bubble-bottom { bottom: 30px; } .bubble-middle-after { left: calc(50% - 8px); } .bubble-middle-before { left: calc(50% - 10px); } .bubble-bottom-before { bottom: -8px; } .bubble-bottom-after { bottom: -12px; border-width: 12px 12px 0; border-style: solid; border-color: #FFFFFF transparent; } .bubble-after { content: ''; position: absolute; display: block; } .bubble-before { content: ''; position: absolute; border-style: solid; border-color: #bababf; border-width: 13px 13px 0; background-color: white; display: block; box-shadow: 0px 0px 5px #6bcded; width: 0px; height: 10px; z-index: -1; transform: rotate(45deg); -moz-transform: rotate(45deg); -ms-transform: rotate(45deg); -o-transform: rotate(45deg); -webkit-transform: rotate(45deg); } 

 <div class="bubble-wrapper"> O <div class="bubble bubble-middle bubble-bottom" arrow-horizontal="middle" arrow-vertical="bottom"> <div class="bubble-inner-wrapper"> <div class="bubble-before bubble-middle-before bubble-bottom-before"></div> <span class="ng-scope">How do I center this bubble</span> <div class="bubble-after bubble-middle-after bubble-bottom-after"></div> </div> </div> </div> 

Answer 2

NOTE: Values of 0 shouldn't have units specified.

You could also re-position your bubble absolutely.

.bubble-bottom-after:before { 
  content: 'O';
  position: absolute;
  left: -6px;
}

---

 .smart-bubble-wrapper { position: relative; margin-top: 100px; margin-left: 100px; } .bubble-bottom-after:before { content: 'O'; position: absolute; left: -6px; } .bubble { display: block; position: absolute; padding: 10px; background: #FFF; -webkit-border-radius: 10px; -moz-border-radius: 10px; border-radius: 10px; border: solid 1px #bababf; box-shadow: 0 0 5px #6bcded; left: -105px; color: #333; } .bubble-bottom { bottom: 30px; } .bubble-middle-after { left: calc(50% - 8px); } .bubble-middle-before { left: calc(50% - 10px); } .bubble-bottom-before { bottom: -8px; } .bubble-bottom-after { bottom: -12px; border-width: 12px 12px 0; border-style: solid; border-color: #FFF transparent; } .bubble-after { content: ''; position: absolute; display: block; } .bubble-before { content: ''; position: absolute; border-style: solid; border-color: #bababf; border-width: 13px 13px 0; background-color: white; display: block; box-shadow: 0 0 5px #6bcded; width: 0; height: 10px; z-index: -1; transform: rotate(45deg); -moz-transform: rotate(45deg); -ms-transform: rotate(45deg); -o-transform: rotate(45deg); -webkit-transform: rotate(45deg); } 

 <link href="https://necolas.github.io/normalize.css/4.1.1/normalize.css" rel="stylesheet"/> <div class="smart-bubble-wrapper"> <div class="bubble bubble-middle bubble-bottom" arrow-horizontal="middle" arrow-vertical="bottom"> <div class="bubble-before bubble-middle-before bubble-bottom-before"></div> <span class="ng-scope">How do I center this bubble</span> <div class="bubble-after bubble-middle-after bubble-bottom-after"></div> </div> </div>