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The Bottom-Up Approach

In the bottom-up approach, Mozilla decides what documents to merge by consulting a database in the chrome. The chrome is a set of directories inside the Mozilla install area. The bottom-up system requires that both master and overlay documents be installed in the chrome.

The most common example of bottom-up design is the Mozilla web browser, which includes a set of files packaged together that make up the browser application. Many programmers are aware of this package of files. When those programmers develop browser add-ons, they include overlays that the bottom-up system merges with the Mozilla browser window. The GUI elements in those overlays then appear prominently in that browser window. This strategy allows those add-ons to be exposed to the user as part of a familiar interface.

A simple example of such an add-on is the DOM Inspector. It's available by default in the Mozilla browser, but not in Netscape 7.0, which is also based on the Mozilla platform. There's no DOM Inspector menu item anywhere in the Netscape Navigator 7.0 browser, but the DOM Inspector can be installed later. After it's installed, a menu item appears on the Tools, Web Development menu in the Navigator window. This menu item is the content of a new overlay, delivered with the DOM Inspector application.

It's not mandatory to integrate overlays with the Mozilla browser. Any known XUL window can act as a master document and be the integration point for any overlay.

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