Visual vs. Layout Viewport Demo
Use the viewport on the right to scroll and zoom around. The viewport on the left displays how the browser sees the two viewports. Note how the
position: fixed navbar at the top behaves under pinch-zoom between implementations. Disclaimer: This is the result of my research and a crude
demo, the behavior for implementations (particularily non-Chrome) may not capture all their subtleties.
What the browser sees
What the user sees
Use mouse to pan around (as if using a touch-screen). Hold shift to pinch-zoom in/out.
The layout viewport is unchanged under pinch-zoom - the position: fixed bar does not follow the visual viewport when zoomed in.
Essentially the same model as Chrome. One major difference, not visible here, is that Chrome sizes the layout viewport to be the
size of the visual viewport at minimum-scale. Edge sizes it to be the size of the visual viewport at scale=1. This means Edge's
visual viewport can be larger than the layout viewport.
Firefox does not distinguish between the visual and layout viewport. As the user pinch-zooms, the layout viewport shrinks with
the visual viewport. The practical effect of this is that position: fixed elements stick to the visual viewport and there's no
way to scroll them if they overflow the visual viewport.
Something of a hybrid. When the user first starts to pinch-zoom, the layout viewport shrinks along with the visual viewport so
that the position: fixed element stays in the visual viewport. However, after crossing some threshold, the layout viewport
stops scaling and only the visual scales. When scrolling, both viewports are scrolled at the same time proportionally.