Bug 227486 – Pinch-zooming gets stuck sometimes

• Component: Scrolling
• Status: NEW

When I pinch to zoom, it doesn't always work correctly. Sometimes, it doesn't work at all, sometimes it works as expected, and sometimes it gets stopped about halfway through zooming. It's hard to pinpoint what exactly triggers these different conditions. It happens on both zoom in and zoom out, but it's more common on zoom out.

It's a bit easier to trigger by zooming, moving your fingers elsewhere on the trackpad, and doing the opposite zooming gesture.

This is reminiscent of https://bugs.webkit.org/show_bug.cgi?id=203132 that I filed last year but it's slightly different.

[Simon Fraser (smfr)]

We're going to need a bit more info. Does this happen on all pages? Does this happen on different hardware (if you can test)?

Hi. Sorry for the lack of info. It's hard to reproduce. It does happen on any page and I've tried it on an Intel (Big Sur) and M1 (Monterey) Mac mini with included Safari and STP. It happens in all cases but it seems that:

It happens in STP more often on Big Sur.
It happens in Safari more on Monterey.

The sites I've been testing it the most on are https://www.wireshark.org/download.html,  https://www.tripadvisor.com/VacationRentalReview-g60842-d19335162-Magical_Memories_Cabin_Gatlinburg_Pigeon_Forge_Hot_Tub_Pool_Table_Fireplace-Gatlinburg.html, and https://www.tripadvisor.com/Rentals but that's mostly just because I randomly picked them to try. They all exhibit the problem, though.

It's like it gets stuck between wanting to zoom the page and pan it. Also, sometimes when I zoom in I will end up on a random part of the page. 

The best reproduction steps I can give are, pinch on the pages a lot and it'll happen eventually. It's not every time but it's common enough to be an issue.

Created attachment 432540 [details]
Intel Big Sur with Safari zooming to random places

Created attachment 432544 [details]
M1 Monterey with STP showing missed gestures and random zoomed location

Almost every time in the s video that my mouse cursor slides up and to the left is a missed pinch gesture. Also notice the random zoomed spot at the end.

[Simon Fraser (smfr)]

Do you happen to have viewed a PDF in the same tab before this happens?

No, on either Mac. These are fresh tabs. I have other tabs open but none are PDFs.

[Radar WebKit Bug Importer]

<rdar://problem/82975818>