Expand a div to fill the remaining width

The solution to this is actually very easy, but not at all obvious. You have to trigger something called a "block formatting context" (BFC), which interacts with floats in a specific way.

Just take that second div, remove the float, and give it overflow:hidden instead. Any overflow value other than visible makes the block it's set on become a BFC. BFCs don't allow descendant floats to escape them, nor do they allow sibling/ancestor floats to intrude into them. The net effect here is that the floated div will do its thing, then the second div will be an ordinary block, taking up all available width except that occupied by the float.

This should work across all current browsers, though you may have to trigger hasLayout in IE6 and 7. I can't recall.

Demos:

• Fixed Left: http://jsfiddle.net/A8zLY/5/
• Fixed Right: http://jsfiddle.net/A8zLY/2/

div {
  float: left;
}

.second {
  background: #ccc;
  float: none;
  overflow: hidden;
}

<div>Tree</div>
<div class="second">View</div>

I just discovered the magic of flex boxes (display: flex). Try this:

<style>
  #box {
    display: flex;
  }
  #b {
    flex-grow: 100;
    border: 1px solid green;
  }
</style>
<div id='box'>
 <div id='a'>Tree</div>
 <div id='b'>View</div>
</div>

Flex boxes give me the control I've wished css had for 15 years. Its finally here! More info: https://css-tricks.com/snippets/css/a-guide-to-flexbox/

Use the CSS Flexbox flex-grow property to fill the remaining space.

html, body {
  height: 100%;
}
body {
  display: flex;
}
.second {
  flex-grow: 1;
}

<div style="background: #bef;">Tree</div>
<div class="second" style="background: #ff9;">View</div>