Hiroyuki Ikezoe
88ac7f97eb
getBoundingClientRect() coordinates are stored internally as 32-bit floats (CSSPoint), whereas scrollTop is reported as a double. At the multi-million-pixel scroll offsets this test exercises, one float ULP exceeds a CSS pixel, so the two values unavoidably diverge by an amount that grows with the coordinate magnitude. The fixed epsilon did not account for this and failed once the divergence passed ~1px (exposed by enabling layout.disable-pixel-alignment in bug 2048146). Scale the tolerance with the float ULP at the compared magnitude. Differential Revision: https://phabricator.services.mozilla.com/D308149
64 lines
1.9 KiB
HTML
64 lines
1.9 KiB
HTML
<!DOCTYPE html>
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<html>
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<script src="/tests/SimpleTest/SimpleTest.js"></script>
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<link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
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<style>
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#marker {
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width: 1px;
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height: 1px;
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}
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#outer {
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height: 1000px;
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background: #a0ffff;
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overflow: scroll;
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}
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#inner {
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height: 16593200px;
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background: #ffa0a0;
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}
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.log {
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white-space: pre;
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}
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</style>
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<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1831148">Mozilla Bug 1831148</a>
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<div id="marker"></div>
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<div id="outer">
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<div id="inner"></div>
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</div>
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<script>
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SimpleTest.waitForExplicitFinish();
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// getBoundingClientRect() returns coordinates that are stored internally as
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// 32-bit floats (CSSPoint), whereas scrollTop is reported as a double. At the
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// multi-million-pixel scroll offsets exercised here, one float ULP exceeds a
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// CSS pixel, so the two unavoidably diverge by an amount that grows with the
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// magnitude. Returns one float ULP at the given magnitude (23-bit mantissa).
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function floatEpsilon(value) {
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const magnitude = Math.abs(value);
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if (magnitude === 0) {
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return 0;
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}
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return Math.pow(2, Math.floor(Math.log2(magnitude)) - 23);
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}
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addLoadEvent(() => {
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const interval = 20;
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const increment = outer.scrollHeight / interval;
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const offset = marker.getBoundingClientRect().bottom;
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for (let i = 0; i < interval; i++) {
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outer.scrollTop = i * increment;
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console.log(outer.scrollTop);
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// Shift to account for viewport coordinate shift.
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const bcrTop = -inner.getBoundingClientRect().top + offset;
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// Tolerate device-pixel snapping plus the float quantization of the BCR
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// (a few ULPs across the negate/offset arithmetic), which scales with the
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// scroll offset.
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const epsilon = devicePixelRatio + 2 * floatEpsilon(outer.scrollTop);
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isfuzzy(bcrTop, outer.scrollTop, epsilon, "scrollTop and BCR top match");
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}
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SimpleTest.finish();
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});
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</script>
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