I recently discovered that your custom master page may affect the Gantt view of a SharePoint list. I have seen the colored task bars extend past the framed container of the chart when a custom master page and CSS is applied to the site. If this happens to you, you can try the following:
- Systematically strip out the CSS and HTML to identify what code is causing the problem. Be sure to make backups of your files first.
- Adjust the DOCTYPE tag that you are using. When this happened to me, I had to go way back and use the following DOCTYPE to get the Gantt view to show correctly: <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
And finally, add this on to your list of items to test while developing custom master pages for SharePoint!