Custom content can be placed in the PanelBar's headers, however you are limited to using inline elements only (span, a, em, strong, img, etc.). Using block elements such as div and p is theoretically possible, but it would make the HTML markup invalid from web standards point of view.
Here is a simple example. If you apply enough custom classes to the custom HTML tags, you will be able to style them, according to your preferences.
Also note that custom clickable elements inside the header must prevent event bubbling, so that the PanelBar does not trigger item expand/collapse.
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