The code in this directory implements the Frame concept in Blink, which is the fundamental container for a web document and its associated properties.
Blink employs throttling mechanisms to reduce resource consumption for out-of-viewport and background frames, improving performance and battery life for the foreground content. There are two primary forms of throttling, both triggered when a frame's viewport intersection becomes empty.
Render throttling aims to reduce rendering work for cross-origin iframes that are outside of the viewport. When a cross-origin iframe is scrolled out of view, Blink stops running requestAnimationFrame, style, layout, and paint for that frame on every BeginMainFrame. Same-origin iframes are not subject to this throttling. Script execution continues in a render-throttled frame. The implementation details can be found in FrameView::UpdateRenderThrottlingStatus and are further explained in the Render Throttling design doc.
Timer throttling is a mechanism that reduces the frequency of timers for out-of-viewport frames. This affects setTimeout and setInterval timers, limiting their execution to save resources. While related to a frame's visibility status, the specifics of timer throttling are part of a broader set of scheduling policies. You can find more information about timer throttling and other scheduling behaviors in Task Scheduling in Blink. The visibility update that triggers this is in FrameView::UpdateFrameVisibility.