You know the scenario. Customer wants a SharePoint site that doesn't look like SharePoint, so you create a custom CSS, override a bunch of styles, everybody is happy, end of story. Well, almost end of story. The slideshow navigation disappeared, you say? How can that be? All we did was use some different colors and graphics. This issue drove me crazy for a while, but I finally found the cause.
One of my customers is using a "no code" approach to SharePoint 2007, knowing that SharePoint 2010 is on the horizon. So we have the classic scenario where we write some custom CSS, override the styles in core.css, deploy our master page and life is good. Unless you want the system pages to look like your content pages. Since we are "no code", we cannot deploy a branding feature. The publishing feature is enabled on the site collection, so we used the option of an alternate CSS to override a bunch of styles and come up with a fairly seamless experience between system pages (site settings, all site content, etc.) and our content pages. However, ever time we apply the alternate CSS, the navigation controls on the Picture Library's slideshow view disappear. Remove the alternate CSS and the controls come back, even with the same CSS being the "default" CSS for the master page. The alternate CSS is a must, though, in order to override system page styles with a "no code" approach. So I started digging through 6000 lines of CSS. The culprit turned out to be the following lines in our CSS...
I removed the "form" tag and everything went back to working properly. Don't know why it caused an issue with the slideshow (but nowhere else), but it did. I ended up splitting my originall CSS into two files. One contains the styles for the content pages and is applied via the master page's default CSS property. The second contains the style overrides for the system pages and is applied via the alternate CSS option (under Site Settings --> Master Page). Hopefully this saves someone some time and aggravation.