Browsers have two distinct zoom modes in the wild:In Firefox Mobile text is usually re-flowed on pinch zoom. Sometimes the text layout does spill out of the screen but adjusting the pinch zoom a bit remedies that. This is an observation as an end-user. I have read neither the spec's around this nor source code.
1) The mobile browser zoom-and-pan mode (called "pinch zoom" in the cssom-view spec): zooming doesn't change layout, and simply performs a draw-time upscale or something close to it (font rasterization here is complicated). The size of the viewport (e.g. as measured by window.innerWidth) does not actually change in this mode. Instead, the page spills out of the viewport if you zoom in and you can pan around it.
2) The desktop browser page zoom mode (called "page zoom" in the spec): zooming changes the size of a CSS pixel as measured in device pixels (and hence the size of the viewport) then redoes the page layout using the new viewport size. Effectively, this corresponds more or less (again, with some font rasterization complications) to resizing the window and then doing a paint-time upscale back to the original size. This can change where linebreaks happen and whatnot. Typically the resulting layout will not spill out of the viewport horizontally.
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