html - CSS to create curved corner between two elements?

Question

My UI has an unordered list on the left. When a list item is selected, a div appears on the right of it. I'd like to have a curved outer corner where the <li> and the <div> meet. Some people call this a negative border radius or an inverted corner. See the white arrow in the image below.

To extend the blue <li> to the edge of the <ul>, I'm planning to do something like this:

li { 
    right-margin: 2em; 
    border-radius: 8px; 
}

li.active { 
    right-margin: 0; 
    border-bottom-right-radius: 0; 
    border-top-right-radius: 0;
}

Is there a better way to extend the <li> to the edge of the <ul>? Obviously, I'll include the webkit and mozilla border radius CSS as well.

The main thing I'm unsure about is that outer corner underneath the bottom right corner of the active <li>. I have some ideas, but they seem like hacks. Any suggestions?

NOTE that the <ul> is indicated in grey, but it would be white in the real design. Also, I'm planning to use Javascript to position the <div> correctly when an <li> is selected.

Answer 1

Well, as it turns out, I managed to solve the problem myself. I hacked together a demo -- check it out.

Essentially, several additional DOM elements are required and a fair amount of CSS. And as mentioned in the link provided by @Steve, a solid background is required. I don't believe there is any way to do this over a gradient background or other pattern.

I ended up with HTML like this:

ul.selectable {
  padding-top: 1em;
  padding-bottom: 1em;
  width: 50%;
  float: left;
}
ul.selectable li {
  margin: 0 3em 0 4em;
  border-radius: 8px;
  -webkit-border-radius: 8px;
  -khtml-border-radius: 8px;
  -moz-border-radius: 8px;
}
ul.selectable li.active {
  margin-right: 0;
}
ul.selectable li.active dl {
  background-color: #4f9ddf;
}
ul.selectable li dt {
  background-color: #dfd24f;
  padding: 1em;
  margin-left: -2em;
  margin-right: -2em;
  -webkit-border-radius: 8px;
  -khtml-border-radius: 8px;
  -moz-border-radius: 8px;
  border-radius: 8px;
}
ul.selectable li dd {
  padding: 0.25em;
  background-color: #fff;
}
ul.selectable li.active dt {
  background-color: #4f9ddf;
  margin-right: 0;
  -webkit-border-top-right-radius: 0;
  -webkit-border-bottom-right-radius: 0;
  -khtml-border-top-right-radius: 0;
  -khtml-border-bottom-right-radius: 0;
  -moz-border-radius-topright: 0;
  -moz-border-radius-bottomright: 0;
  border-top-right-radius: 0;
  border-bottom-right-radius: 0;
}
ul.selectable li.active dd.top {
  -webkit-border-bottom-right-radius: 8px;
  -khtml-border-bottom-right-radius: 8px;
  -moz-border-radius-bottomright: 8px;
  border-bottom-right-radius: 8px;
}
ul.selectable li.active dd.bot {
  -webkit-border-top-right-radius: 8px;
  -khtml-border-top-right-radius: 8px;
  -moz-border-radius-topright: 8px;
  border-top-right-radius: 8px;
}
div.right {
  float: left;
  padding-top: 3em;
  width: 50%;
}
div.content {
  height: 15em;
  width: 80%;
  background-color: #4f9ddf;
  padding: 1em;
  -webkit-border-radius: 8px;
  -khtml-border-radius: 8px;
  -moz-border-radius: 8px;
  border-radius: 8px;
}

<ul class="selectable">
  <li>
    <dl>
      <dd class="top"></dd>
      <dt>Title</dt>
      <dd class="bot"></dd>
    </dl>
  </li>
  <li class="active">
    <dl>
      <dd class="top"></dd>
      <dt>Title</dt>
      <dd class="bot"></dd>
    </dl>
  </li>
  <li>
    <dl>
      <dd class="top"></dd>
      <dt>Title</dt>
      <dd class="bot"></dd>
    </dl>
  </li>
</ul>
<div class="right">
  <div class="content">This is content</div>
</div>