feat(html): expose the zoom every view opens at to the host

[andiwand]

🤖 Generated with Claude Code

The fit from #719 landed a scale on the body and nothing could move it afterwards. An app wanting a zoom control had to reach for the platform's own — WKWebView.pageZoom, WebView.zoomBy — which differ per platform, are inert for an embedder rendering us in a frame, and know nothing of the fit. That is the thing #719 just climbed out of.

The api

Written into the odr object every view already carries:

odr.getZoom()                    // effective, 1 is actual size
odr.setZoom(value, focus)        // pins it, returns what was applied
odr.adjustZoom(factor, focus)    // multiplicative, pins it
odr.resetZoom(focus)             // back to following the fit
odr.isZoomFitted()
odr.onZoomChange = function (zoom, fitted) {}

Clamped to [0.1, 10]; a value that is not a number leaves the zoom alone. onZoomChange fires on a resize too, since the effective zoom moves while fitted without anyone calling a setter.

The state behind it

One nullable number: a pinned zoom, or null while the view follows the fit — not a multiplier over the fit. A phone rotating with a 2× multiplier compounds it against the wider fit and overshoots, where a pinned zoom keeps the text the size the reader set. While pinned, the fit is not measured at all, so a resize costs nothing until resetZoom().

isZoomFitted() is the one bit a toolbar cannot derive from the number: "fitted at 0.72" and "pinned at 0.72" read the same otherwise.

Focal point

focus is the point the zoom is centred on — a pinch's midpoint — given as {x, y} or any object carrying clientX/clientY, so a touch or mouse event can be handed straight over. The anchor that holds the reading position now pins the horizontal too, but only when a point asked for it: otherwise the page column centres itself and holding x would fight that.

Css says what the view opens at

So a page under a strict Content-Security-Policy opens right with no script: --odr-fit is the factor, or auto where only the view can measure it, and --odr-zoom plus body{zoom} carry what is applied. HtmlConfig::initial_zoom writes that and reaches through all four bindings (python, jni, apple, wasm). Printing resets both — paper has its own geometry, and !important beats the inline zoom the script writes.

Coverage

Documents, pdf, text, xml, the archive listing and images. An image is fitted by css rather than by a body zoom (100% of a zoomed body is the viewport again), so its max-width puts the factor back to grow with it. The media view opts out — zooming a <video> is not meaningful — and the font preview ships no resources to hang a script on.

Verified

html_common and html suites pass, plus the 306 html-output tests. In Chrome, framed and top-level:

• the fit tracks resizes while fitted (0.385 → 0.771 at double width) and holds at 1.5 while pinned
• resetZoom() re-measures against the current width
• the static css factor is read back by the script instead of being re-measured
• the reading position drifts by ~0 across zoom steps, including back-to-back changes
• a focal point stays within half a pixel over zoom in, further in, and out
• an image doubles its rendered width at 2×

Not in here

• Pinch gestures. This is the lever, not the gesture: in a frame a two-finger gesture still goes to the top-level page. A built-in handler (touch-action plus touchmove, and WebKit's non-standard gesturechange) needs real-device testing on both platforms and can break panning if it is wrong.
• Reference output. Every touched view gains a small <style> in its head, so the pinned output drifts — same as feat(html): fit paged output to the viewport, and hold the reading position #719, which did not advance the pointer either.

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