There exists a common misconception z-index will solve all of your DHTML stacking problems. While this is true for DHTML elements, the assumption falls down in the face of Internet Explorer and the way this browser treats "windowed" components like drop down lists based on the select tag. Flash objects aren't immune but for different reasons. Firefox will honour stacking orders for all elements but this is not the case when you're dealing with IE6/7. Windowed elements maintain a different stacking order than DHTML elements (they're added to a different plane than your windows elements) and therein lies the problem.
IE7 is better than IE6 when it comes to the select element but Flash requires some additional finessing, which I'll detail below. If you're still supporting IE6, consider using an IFrame (perhaps dynamically positioned) between your windowed component and your DHTML object to mask the underlying windowed component. The concept is generally referred to as an "IFrame shim". Don't forget, IFrames can be configured with a transparent background; either way, simply point the IFrame URL at nothing. The IFrame is no longer a windowed element as of IE5.5; although some people don't like IFrames for security and because they're frames, in this case the IFrame is your friend because it bridges the gap between windowed elements and windowless elements. The one main gotcha with this approach is how the user interacts with an IFrame: button clicks and mouse actions will be sent to the IFrame and the underlying content won't be interactive while the IFrame is visible.
Flash--being Flash and an ActiveX-wrapped plugin--is a different beast. IE places embedded content in a DHTML layer above all other layers by default and this is not always where you want your Flash content. On the westernaustralia.com home page, a fancy combination of DHTML and Flash elements known as the Tourism Australia overlay sits above the wa.com Flash banner (it's only visible to international site visitors); the DHTML-based navigation likewise has to sit above the central "experience panel" and the global site selector drop down in the right column.
To reign in your Flash objects and their stacking order you need to use an extra parameter named wmode:
<param name="wmode" value="opaque" />
By doing so the Flash object and surrounding DHTML elements will team up to honour z-index styles as you intend. It's also a good idea to add a corresponding wmode="opaque" attribute to your element tag. Opaque and transparent modes may compromise your animations and video--we haven't had any problem on our sites, however, and use both animations (the wipe) and video.
Here are a few additional tips for dealing with Flash:
- Don't rely solely on the embed tag. Use the object tag for IE and nest an embed tag within it for Mozilla and everyone else.
- Always explicitly configure the wmode parameter as a child of the object tag and an attribute of the element tag.
- Avoid the wmode "transparent" setting unless your Flash movie actually has transparent sections. Use "opaque" in most cases to increase rendering performance. The default is "window".
- Consider a Flash alternative like Silverlight ;-)
Microsoft's windowless vs windowed summary:
Adobe's semi-useful KB article about wmode:
How to create an IFrame shim:
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