Summary
The "Jump to a Section" feature in Pages is available for clickable elements such as links, images, buttons, and the Navigation block. The "Select a Section" submenu defines the jump destination. Clicking on the submenu should expand a dropdown list, but sometimes it does not work and is greyed out.
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Resolution
Define an ID for applicable Page Sections by clicking on a section and adding a value to its ID field.
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Cause
The "Select a Section" submenu will not expand if no Sections have a defined ID. There must be at least one Section with an ID in order for the submenu to populate and react on click.