Send hidden and visible system visibility updates from explicit tab
switch paths instead of relying only on widget show and hide events. Tab
containers can keep inactive web views mapped, which leaves documents
visible from WebContent's perspective and lets animated images keep
running in background tabs.
Use AppKit window occlusion state rather than the app-wide active tab.
That keeps still-visible background windows in the visible document
state. Avoid resending duplicate visibility updates from
ViewImplementation so existing widget visibility notifications remain
harmless.
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