236312 2022-02-08 10:18:41 -0800 Rapid changes in scroll position can cause JS-exposed scroll offsets to be stale 2023-02-23 05:50:51 -0800 1 1 1 Unclassified WebKit Scrolling Safari 15 iPhone / iPad Unspecified NEW InRadar P2 Major --- 1 jack webkit-unassigned hi simon.fraser webkit-bug-importer oldest_to_newest 1838750 0 jack 2022-02-08 10:18:41 -0800 Set the scroll position via JavaScript and then immediately check it and it seems to work correctly but under heavy load it actually REVERTS to a previous scroll position on the next requestAnimationFrame()! From what I can tell, it's definitely a bug. Here is the proof (you must run this on iOS Safari in the /debug/ window of CodePen so that it's not in an iframe): https://codepen.io/GreenSock/pen/16c435b12ef09c38125204818e7b45fc?editors=0010 Again, this only happens under heavy load. I understand JS runs on a separate thread from scrolling but all of this is happening in JS world so the regression of the scroll position doesn't make sense to me. Can anyone explain why this is happening and how we can work around it? 1841547 1 webkit-bug-importer 2022-02-14 19:54:25 -0800 <rdar://problem/88943486> 1841952 2 452091 gsnedders 2022-02-15 14:44:11 -0800 Created attachment 452091 Testcase I couldn't get the codepen to load in debug mode, so here's it attached. <rdar://88342742> 1841976 3 simon.fraser 2022-02-15 15:22:00 -0800 Scrolling on iOS involves an asynchronous round-trip to a different process, and it's possible we're getting a stale scroll position back to JS because of that. I don't know of any workarounds currently. 1936144 4 hi 2023-02-23 05:50:51 -0800 Are there any intentions to look into this? 452091 2022-02-15 14:44:11 -0800 2022-02-15 14:44:11 -0800 Testcase test.html text/html 2258 gsnedders CjwhRE9DVFlQRSBodG1sPgo8aHRtbCBsYW5nPSJlbiIgPgoKPGhlYWQ+CgogIDxtZXRhIGNoYXJz ZXQ9IlVURi04Ij4KICAKCiAgPHRpdGxlPkNvZGVQZW4gLSBpT1MgU2FmYXJpIFNjcm9sbGluZyBC dWc8L3RpdGxlPgogIAogIAogIAogIAo8c3R5bGU+CmJvZHkgewogIGZvbnQtZmFtaWx5OiBzYW5z LXNlcmlmOwp9CiogewogIGJveC1zaXppbmc6IGJvcmRlci1ib3g7Cn0KYm9keSwgaHRtbCB7CiAg bWFyZ2luOiAwOwogIHBhZGRpbmc6IDA7CiAgc2Nyb2xsLWJlaGF2aW9yOiBhdXRvOwogIHRyYW5z aXRpb246IG5vbmU7Cn0KCi5vdmVybGF5IHsKICBwb3NpdGlvbjogZml4ZWQ7CiAgaGVpZ2h0OiAx MDB2aDsKICBwYWRkaW5nOiAyMHB4OwogIGNvbG9yOiByZWQ7Cn0KLmNvbnRlbnQgewogIGhlaWdo dDogODAwdmg7CiAgbGluZS1oZWlnaHQ6IDEwMHZoOwogIGZvbnQtc2l6ZTogNTBweDsKICB0ZXh0 LWFsaWduOiBjZW50ZXI7Cn0KPC9zdHlsZT4KCgo8L2hlYWQ+Cgo8Ym9keSB0cmFuc2xhdGU9Im5v IiA+CiAgPGRpdiBjbGFzcz0ib3ZlcmxheSI+PC9kaXY+CjxkaXYgY2xhc3M9ImNvbnRlbnQiPmlP UyBTYWZhcmkgQnVnPC9kaXY+CiAgICAgIDxzY3JpcHQgaWQ9InJlbmRlcmVkLWpzIiA+Ci8vIFlv dSBNVVNUIHJ1biB0aGlzIGluIC9kZWJ1Zy8gbW9kZSB0byBleHBvc2UgdGhlIGJ1ZyAod29ya3Mg ZmluZSBpbiBpZnJhbWUpLiAKLy8gaHR0cHM6Ly9jb2RlcGVuLmlvL0dyZWVuU29jay9kZWJ1Zy8x NmM0MzViMTJlZjA5YzM4MTI1MjA0ODE4ZTdiNDVmYz9lZGl0b3JzPTAwMTAKLy8gaU9TIG1pc3Jl cG9ydHMgc2Nyb2xsIHBvc2l0aW9uISBTZXQgaXQsIGNoZWNrIGl0IGltbWVkaWF0ZWx5IGFuZCBp dCdzIGNvcnJlY3QgYnV0IHRoZW4gCi8vIG9uIHRoZSBuZXh0IHJlcXVlc3RBbmltYXRpb25GcmFt ZSAob3Igc29tZXRpbWVzIGluIHRoZSBzYW1lIG9uZSksIGl0IFJFVkVSVFMgdG8gdGhlIHByZXZp b3VzIHNjcm9sbCBwb3NpdGlvbiEhIQoKbGV0IGluY3JlYXNlTG9hZCA9IHRydWUsIC8vIGhlYXZp ZXIgbG9hZCBjYXVzZXMgdGhlIHByb2JsZW0gdG8gc2hvdyB1cC4KaXRlcmF0aW9ucyA9IDUwLApj dXJyZW50U2Nyb2xsID0gMCwKaXNDbG9zZUVub3VnaCA9ICh2MSwgdjIpID0+IE1hdGguYWJzKHYx IC0gdjIpIDw9IDI7IC8vIFNhZmFyaSBpcyBvZnRlbiAxIHBpeGVsIG9mZiwgc28gd2UgZ2l2ZSBp dCAyIHBpeGVscyBsZWV3YXk7CgovLyB0aGlzIGZ1bmN0aW9uIGp1c3QgY2hhbmdlcyB0aGUgc2Ny b2xsIHBvc2l0aW9uIHRvIGEgcmFuZG9tIG9uZSwgYnV0IGJlZm9yZSBpdCBkb2VzIHNvLCBpdCBj aGVja3MgdG8gc2VlIGlmIHRoZSBwcmV2aW91cyBzY3JvbGwgcG9zaXRpb24gdGhhdCB3YXMgc2V0 IGlzIHN0aWxsIHdoYXQgaXQgd2FzIHN1cHBvc2VkIHRvIGJlIChleHBvc2UgdGhlIGJ1ZykKZnVu Y3Rpb24gY2hhbmdlU2Nyb2xsKCkgewogIGNoZWNrU2Nyb2xsKCk7IC8vIG1ha2Ugc3VyZSBpdCBt YXRjaGVzIHdoYXQgd2Ugc2V0IHByZXZpb3VzbHkKICBjdXJyZW50U2Nyb2xsID0gTWF0aC5yb3Vu ZChNYXRoLnJhbmRvbSgpICogODAwKTsgLy8gcGljayBhIHJhbmRvbSBzY3JvbGwgcG9zaXRpb24K ICB3aW5kb3cuc2Nyb2xsVG8oMCwgY3VycmVudFNjcm9sbCk7IC8vIGdvIHRoZXJlCiAgaWYgKGl0 ZXJhdGlvbnMtLSkgey8vIG9ubHkgZG8gdGhpcyBmb3IgYSBjZXJ0YWluIG51bWJlciBvZiBpdGVy YXRpb25zCiAgICByZXF1ZXN0QW5pbWF0aW9uRnJhbWUoY2hhbmdlU2Nyb2xsKTsKICB9Cn0KCi8v IE9OTFkgV09SS1MgSUYgV0UgVVNFIEFTWU5DIEFORCBBV0FJVCBUUlVFIEhFUkUhISEKZnVuY3Rp b24gY2hlY2tTY3JvbGwoKSB7CiAgaWYgKGluY3JlYXNlTG9hZCkgey8vIGp1c3QgY2hld3MgdXAg Q1BVIHRpbWUuCiAgICBmb3IgKGxldCBpID0gMDsgaSA8IDk5OTk5OTk7IGkrKykgewogICAgICBs ZXQgZHVtbXlWYWx1ZSA9IE1hdGgucmFuZG9tKCkgKiBNYXRoLnJhbmRvbSgpOwogICAgfQogIH0K ICBpZiAoIWlzQ2xvc2VFbm91Z2god2luZG93LnBhZ2VZT2Zmc2V0LCBjdXJyZW50U2Nyb2xsKSkg ey8vIGRvZXMgaXQgc3RpbGwgbWF0Y2g/CiAgICBkb2N1bWVudC5xdWVyeVNlbGVjdG9yKCIub3Zl cmxheSIpLmlubmVySFRNTCArPSAiRVJST1IhICIgKyB3aW5kb3cucGFnZVlPZmZzZXQgKyAiICE9 PSAiICsgY3VycmVudFNjcm9sbCArICI8YnI+IjsKICB9Cn0KCmNoYW5nZVNjcm9sbCgpOwogICAg PC9zY3JpcHQ+CgogIAoKPC9ib2R5PgoKPC9odG1sPgogCgo=