IE display transparency bug on height > 4096px?

Question

I was working on a JavaScript dialog with a transparent background overlay when I ran into a problem on large pages.

If the page was large, the transparent overlay would be a solid colour (i.e. no longer transparent). I did some testing and found this only happened in the overlay was greater than 4096 pixels high (hmmm, suspicious, that's 2^12).

Can anyone verify this issue? Have you seen a work-around?

Here's my test code (I'm using Prototype):

<style>
.overlayA { 
    position:absolute;
    z-index:10;
    width:100%;
    height:4095px;
    top:0px;
    left:0px;
    zoom: 1;
    background-color:#000;
    filter:alpha(opacity=10);
    -moz-opacity:0.1;
    opacity:0.1;
}

.overlayB { 
    position:absolute;
    z-index:10;
    width:100%;
    height:4097px;
    top:0px;
    left:0px;
    zoom: 1;
    background-color:#000;
    filter:alpha(opacity=10);
    -moz-opacity:0.1;
    opacity:0.1;
}
</style>
<div style="width:550px;height:5000px;border:1px solid #808080">
    <a href="javascript:// show overlay A" onclick="Element.show('overlayA')">Display A = 4096h</a>
    <br /><a href="javascript:// show overlay B" onclick="Element.show('overlayB')">Display B = 4097h</a>
</div>
<div id="overlayA" onclick="Element.hide(this)" class="overlayA" style="display:none"></div>
<div id="overlayB" onclick="Element.hide(this)" class="overlayB" style="display:none"></div>

Answer 1

How about making the overlay the size of the window instead of the size of the page, and moving it up or down on scroll.

Answer 2

You are operating at the edge already (that's huge...) so I don't know that MS would classify it as a bug or 'fix' it even if it was.

You might need to break it up into smaller overlay DIVs.

Answer 3

Why wouldn't you postion the overlay fixed?
That way it wouldn't have to be as big as the whole page content.

Simply doing:

 #Overlay{
  position:fixed;
  left:0px;
  top:0px;
  height:100%;
  width:100%;
  rest of declarations
 }

Just make sure it's parent is the document and the document has a width and height of 100%. That way you should be good with a much smaller overlay.

THe posotion:fixed will make sure the overlay is positioned relative to the viewport. Thus its always displayed in the top left corner.

Answer 4

Since you have an opacity filter on the CSS I believe you are indirectly using DirectShow under the covers for alpha blending and image composition. DirectShow uses DirectX textures, which have a 4096x4096 pixel limit for DX9, which would explain this erratic behavior.

Answer 5

The position:fixed solution is a spotty solution..It is not well supported in IE.

The best thing is to automatically create and append additional transparent elements (with a max height of 2048px to cover XP DX8 which has this issue as well).

Here's the code I used, assuming you already have a floating div solution.

 if(document.getElementById('document_body').scrollHeight > 2048)
 {
  document.getElementById('float_bg').style.height = "2048px";
  document.getElementById('float_bg').style.zIndex = -1;
  count=1;
  total_height=2048;
  while(total_height < document.getElementById('document_body').scrollHeight)
  {
   clone = document.getElementById('float_bg').cloneNode(true);
   clone.id = 'float_bg_'+count;
   clone.style.zIndex = -1;
   //clone.style.backgroundColor='red';
   clone.style.top = (count*2048)+"px";
   document.getElementById('float_el').insertBefore(clone,document.getElementById('float_bg'));
   count++;    

   this_add = 2048;
   if((total_height + 2048) > document.body.scrollHeight)
   {
    clone.style.height = (document.body.scrollHeight - total_height);
   }
   total_height += this_add;
  }
 }
 else
 {
   document.getElementById('float_bg').style.height = document.body.scrollHeight + "px";
 }