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== Introduction ==
The '''Async Pan/Zoom module (APZ)''' (formerly called "Async Pan<sup>1</Zoom Controller" or "APZC") sup> is a platform component that allows panning and zooming to be performed asynchronously (on the compositor thread rather than the main thead).
For '''zooming''', this means that the APZ reacts to a pinch gesture immediately and instructs the compositor to scale the already-rendered layers at whatever resolution they have been rendered (so e.g. text becomes more blurry as you zoom in), and meanwhile sends a request to Gecko to re-render the content at a new resolution (with sharp text and all).
For '''panning''', this means that the APZ asks Gecko to render a portion of a scrollable layer, called the "display port", that's larger than the visible portion. It then reacts to a pan gesture immediately, asking the compositor to render a different portion of the displayport (or, if the displayport is not large enough to cover the new visible region, then nothing in the portions it doesn't cover), and meanwhile sends a request to Gecko to render a new displayport. (The displayport can also be used when zooming out causes more content of a scrollable layer to be shown than before.)
<sub>1. This module used to be called '''Async Pan/Zoom Controller (APZC)''', but this was changed to avoid confusion because there is a class called AsyncPanZoomController of which there are now multiple instances.</sub>
== Supported Platforms ==
