边界半径应该剪切内容吗？

This question seems to point to the same defect, apparently this is a bug.

CSS3 border-radius clipping issues

Edit

Eek! BoltClock has mentioned that this is indended so I'll post this other SO question on the topic whilst I also hunt for a spec quote on this. How do I prevent an image from overflowing a rounded corner box with CSS3?

Specification Link

Just for reference, I'll stick the relevant link in - but I can't find anything explicit to the example you've given.

CSS Backgrounds - Rounded Corners

This is what the specifications says, so this is the way it should work. But that doesn't mean that Chrome does it like that.

5.3. Corner Clipping

A box's backgrounds, but not its border-image, are clipped to the appropriate curve (as determined by ‘background-clip’). Other effects that clip to the border or padding edge (such as ‘overflow’ other than ‘visible’) also must clip to the curve. The content of replaced elements is always trimmed to the content edge curve. Also, the area outside the curve of the border edge does not accept mouse events on behalf of the element.

http://www.w3.org/TR/css3-background/#border-radius

For anybody wondering what a fix would be. The easiest way would be to edit the css.

In the example given this would be a suitable fix:

.progressbar { height: 5px; width: 100px; border-radius: 5px; overflow: hidden; }
.buffer { width: 25px; height: 5px; background: #999999; }

<div class="progressbar">
<div class="buffer"></div>
</div>

Is this the expected behavior?

Yes, as crazy as it sounds, it actually is. Here's why:

The default overflow for <div> elements (and most other things) is visible, and the spec says this about overflow: visible:

visible
This value indicates that content is not clipped, i.e., it may be rendered outside the block box.

In turn, §5.3 Corner clipping in the Backgrounds and Borders module says:

A box's backgrounds, but not its border-image, are clipped to the appropriate curve (as determined by ‘background-clip’). Other effects that clip to the border or padding edge (such as ‘overflow’ other than ‘visible’) also must clip to the curve. The content of replaced elements is always trimmed to the content edge curve. Also, the area outside the curve of the border edge does not accept mouse events on behalf of the element.

The sentence that I've emphasized specifically mentions that the overflow value of the box must be something other than visible (that means auto, hidden, scroll and others) in order for the corners to clip its children.

If the box is defined to have visible overflow, which like I said is the default for most visual elements, then the content is not supposed to be clipped at all. And that's why the square corners of .buffer go over the rounded corners of .progressbar.

Consequently, the simplest way to get .buffer to clip within .progressbar's rounded corners is to add an overflow: hidden style to .progressbar, as shown in this updated fiddle.

Semantically speaking, it's best to simply add a border-radius inherit property to the inner div, hence the new class addition:

.buffer {
border-radius: inherit;
}

As a consequence, for others situation, you can preserve the use of overflow: auto if the content overflows your frae and you want to see everything.

The edges and corners of the parent container are covered by quilt elements, so the content of the parent element needs to be cropped, as long as the overflow value is set not visible, for example:

  .parent {
overflow: hidden;
border-radius: 5px;
}