Keep the middle item centered when side items have different widths

Question

Imagine the following layout, where the dots represent the space between the boxes:

[Left box]......[Center box]......[Right box]

When I remove the right box, I like the center box to still be in the center, like so:

[Left box]......[Center box].................

The same goes for if I would remove the left box.

................[Center box].................

Now when the content within the center box gets longer, it will take up as much available space as needed while remaining centered. The left and right box will never shrink and thus when where is no space left the overflow:hidden and text-overflow: ellipsis will come in effect to break the content;

[Left box][Center boxxxxxxxxxxxxx][Right box]

All the above is my ideal situation, but I have no idea how to accomplish this effect. Because when I create a flex structure like so:

.parent {
    display : flex; // flex box
    justify-content : space-between; // horizontal alignment
    align-content   : center; // vertical alignment
}

If the left and right box would be exactly the same size, I get the desired effect. However when one of the two is from a different size the centered box is not truly centered anymore.

Is there anyone that can help me?

Update

A justify-self would be nice, this would be ideal:

.leftBox {
     justify-self : flex-start;
}

.rightBox {
    justify-self : flex-end;
}

Answer 1

If the left and right boxes would be exactly the same size, I get the desired effect. However when one of the two is a different size the centered box is not truly centered anymore. Is there anyone that can help me?

Here's a method using flexbox to center the middle item, regardless of the width of siblings.

Key features:

• pure CSS
• no absolute positioning
• no JS/jQuery

Use nested flex containers and auto margins:

.container {
  display: flex;
}
.box {
  flex: 1;
  display: flex;
  justify-content: center;
}

.box:first-child > span { margin-right: auto; }

.box:last-child  > span { margin-left: auto;  }

/* non-essential */
.box {
  align-items: center;
  border: 1px solid #ccc;
  background-color: lightgreen;
  height: 40px;
}
p {
  text-align: center;
  margin: 5px 0 0 0;
}

<div class="container">
  <div class="box"><span>short text</span></div>
  <div class="box"><span>centered text</span></div>
  <div class="box"><span>loooooooooooooooong text</span></div>
</div>
<p>&#8593;<br>true center</p>

Here's how it works:

• The top-level div (.container) is a flex container.
• Each child div (.box) is now a flex item.
• Each .box item is given flex: 1 in order to distribute container space equally (more details).
• Now the items are consuming all space in the row and are equal width.
• Make each item a (nested) flex container and add justify-content: center.
• Now each span element is a centered flex item.
• Use flex auto margins to shift the outer spans left and right.

You could also forgo justify-content and use auto margins exclusively.

But justify-content can work here because auto margins always have priority.

8.1. Aligning with auto margins

Prior to alignment via justify-content and align-self, any positive free space is distributed to auto margins in that dimension.