Top menu buttons, Site ... Code, each load an
Explorer for the selected button topic.

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For example, Basics button loads Basics Explorer
with its Track Summary page.

Bottom menu buttons, except for "Controls" each send messages
requesting the hosted page to take some action.

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For example, "Sections" button requests page to show list of Page Sections.

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"Controls" button toggles visibility of control block at bottom
left of Explorer view.

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