Question about absolutely positioned block

Is anywhere description
Why absolutely positioned block is not allowed to affect parent element size?

Is it posible you will allow this in CSS3?

In next example I have disadvantages

<div style='position: absolute; width: 300px; border: 1px solid black; margin-top: 10px;'>
  <div style='position: absolute; right: 0px; border: 1px solid red;'>
    LogOut
  </div>
  <span id="mainMenu" style="display:none">File Options Help etc</span>
</div>

You can say that I must use 'float'
<div style='position: absolute; width: 300px; left: 20px; top: 200px ; border: 1px solid black; margin-top: 20px;'>
  <span style='float: right; border: 1px solid red;'>
    LogOut
  </span>
  <span  id="mainMenu" style="display: none">File Options Help etc</span>
</div>

But floats take space from normal flow =(
I want 'LogOut' span does not affect normal flow and affect containing block size
when #mainMenu is hided dynamically