1415786 - Sometimes, Firefox thinks I touched an element that is up to several inches above or below what I actually touched, even if the element is off-screen

Status: UNCONFIRMED

(Core :: Panning and Zooming, defect, P3)

Description

[DavJCobb]

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:56.0) Gecko/20100101 Firefox/56.0
Build ID: 20171024165158

Steps to reproduce:

This bug often occurs when I attempt to touch a hyperlink, or an element that is scripted to react to touch (e.g. a JavaScript-powered collapse/expand box). Touching-and-holding (to access link options) is affected only in the rarest of cases and swiping (to scroll) is never affected, but standard touches (even without scrolling shortly before or after the touch) are affected reasonably often -- maybe once or twice every other day.

There is no specific pattern to this behavior; it has happened to me on just about every website with hyperlinks, provided I've been to that website more than once in my life. The bug has been occurring for, what, two years now? Maybe longer?

In the interest of full disclosure, I did reduce the <ui.touch.radius.topmm> pref from its default value to 3mm, because the default value was too large. Several sites have links or buttons that are in close proximity (e.g. small lines of text), and the default value (5mm) led to Firefox constantly misidentifying what I touched in those cases. A two-millimeter reduction should not lead to this bug, but hey, maybe you want to be thorough if you decide to test this.


Actual results:

Often, Firefox will think that I touched an element that is anywhere from a centimeter to two inches above or below the element that I actually touched. In some cases, Firefox will think that I touched something that is literally off-screen. Elements that Firefox mistakenly believes I've touched will react appropriately to the perceived touch, e.g. links off-screen being clicked and navigating me away from the current page, or scripted elements off-screen collapsing or expanding.

No other apps on my device have this problem, so it is clearly not a hardware or OS issue.


Expected results:

Firefox should recognize that I touched the element that I actually touched, and it should recognize that I have not touched any of the elements that I didn't touch.

Comment 1

[Kartikaya Gupta (email:[e-mail])]

It sounds like this might be a result of async scrolling. Next time you notice it please record the URL of the page and the link you clicked on, so we have a concrete example to work with. Also please try and note whether or not you recently scrolled the page; it might be that the click event is being sent to the content that was at that screen position before your most recent scroll.

Comment 2

[Daniel Bodea [:danibodea]]

Have you noticed this issue happening again? If yes, please take into consideration the request above and try to be more specific about where and how the issue reproduced.

Thank you for your contribution!

Comment 3

[DavJCobb]

This has gotten better since a year ago; it's much less frequent, and tends to occur mainly when the browser is under heavy load (i.e. many tabs open, or an "expensive" page open).

I tend to notice it when browsing reddit with lots of (unloaded) tabs. A reddit mobile post with hundreds of comments will generally put Firefox under enough stress to cause some minor hitches and/or scrolling/tapping problems, though you may need some other tabs open in the background as well. Don't know the URLs of any such posts offhand, though. Wasn't expecting to be asked about this again.