Blink Coordinate Spaces
There are two types of zoom in Chromium: Browser Zoom and Pinch-Zoom.
Browser zoom is what you get by using Ctrl+/- or the zoom buttons in the menu. It changes the size of a CSS pixel relative to a device independent pixel and so it will cause page layout to change. Throughout Blink, Browser-zoom is referred to as “Page Zoom” or “Zoom” more generally.
Pinch-zoom is activated by using a pinch gesture on a touchscreen or touchpad. Throughout Blink and the compositor, pinch-zoom is referred to as “Page Scale”. Pinch-zoom is performed as a post-rendering step, scaling the surface the web page is rendered onto. This means it doesn't interact with layout. Put another way, pinch-zoom doesn't change the relationship between physical pixels and CSS pixels (the `window.devicePixelRatio`) as far as Blink is concerned, even though the user technically sees a different number of physical pixels per CSS pixel.
Physical Pixels: These are the actual pixels on the monitor or screen.
Device Independent Pixels (DIPs): Modern devices sport high density displays (e.g. Apple’s Retina). In order to keep the UI at a comfortably readable size we scale it up based on the density of the display. This scaling factor is called the device pixel ratio in web APIs and device scale factor in Chromium code. A UI will always be the same size in DIPs regardless of how many pixels are actually used to display it. To go from physical pixels to DIPs we divide the physical pixel dimensions by the device scale factor.
Blink implements UI scaling by applying the device scale factor to the browser zoom. As such, code in Blink rarely needs to directly account for device scale factor. Instead, most internal geometry in Blink is done in physical pixels. When interfacing with web content, geometry is converted to CSS pixels by dividing by the browser zoom, which combines both the user's zoom preference (ctrl +/-) and device scale factor.
CSS pixels: CSS defines its own pixel type that is also independent of physical pixels. When there is no Browser-Zoom, Pinch-Zoom, or CSS transforms applied, CSS pixels and DIPs are equivalent. However, zoom can make CSS pixels bigger or smaller relative to DIPs.
Note that the conversion methods between these spaces (in LocalFrameView) do not apply browser zoom, they deal in physical pixels. To convert from locations and sizes expressed in true CSS pixels (as used in web APIs) to Document Content, FrameView Content, or Frame space you must first multiply by the browser-zoom scale factor.
The Frame coordinates of the top level (i.e. main) frame. This frame contains all the other child frames (e.g. elements create frames on a page).
The coordinate space of the visual viewport as seen by the user, in physical pixels. The origin is at the top left corner of the browser view (Window or Screen). The difference between Viewport and RootFrame is the transformation applied by pinch-zoom. This is generally what you'd use to display something relative to the user's Window or Screen.
The final screen space on the user's device, relative to the top left corner of the screen (i.e. if we're in a Window, this will include the window's offset from the top left of the screen). Note that this is in DIPs rather than physical pixels.