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Previously, ion-content only calculated layout offsets on initialization or window resize. This caused content to be covered or obscured if a header or footer's dimensions changed dynamically (e.g. expanding a searchbar) or if they were added conditionally to the DOM.

This implementation adds a ResizeObserver to watch sibling headers/footers for size changes and a MutationObserver to detect when they are added, triggering an automatic recalculation of the --offset-top and --offset-bottom CSS variables.

Fixes ionic-team#26981