Avoid to have to wait to zoom after scrolling

[calixteman]

Allow to zoom with the wheel once the scrolling is finished.
It's now possible to know that thanks to the scrollend event.

Fixes #17707.

[Snuffleupagus]

Keep the previous way to work for browsers not supporting the scrollend event.

Do we actually have to care about such browsers, or could we perhaps take the opportunity to simplify the affected code?

Looking at the MDN compatibility data it seems that almost all modern browsers already support this. The only exception is unsurprisingly Safari, which has always been the "worst" browser to attempt to support. (Historically it's often been quite slow at implementing new web-platform features, and over time we've seen a large number issues that only affect Safari.)

[calixteman]

Keep the previous way to work for browsers not supporting the scrollend event.

Do we actually have to care about such browsers, or could we perhaps take the opportunity to simplify the affected code?

Looking at the MDN compatibility data it seems that almost all modern browsers already support this. The only exception is unsurprisingly Safari, which has always been the "worst" browser to attempt to support. (Historically it's often been quite slow at implementing new web-platform features, and over time we've seen a large number issues that only affect Safari.)

I mostly added this extra code because of:
https://github.com/mozilla/pdf.js/wiki/Frequently-Asked-Questions#legacy-build
and Chrome 92+, when scrollend is available in Chrome 114+.
That said if you agree, I'm fully ok to remove this extra stuff.

[Snuffleupagus]

That said if you agree, I'm fully ok to remove this extra stuff.

My reasoning for removing the old code is:

• We've never guaranteed that every feature will be available in older browsers, since some things cannot be polyfilled.
• These changes don't apply to the PDF.js library, but only to the default viewer (which is developed for Firefox).
• This was intended as nothing more than a work-around for an old Mac-only issue, where the user is pressing the zoom-modifier key during scrolling.

[Snuffleupagus]

Don't we also need to ensure that the new event listeners are removed on "blur" for the mainContainer, and also reset this._isScrolling = false; then?

[calixteman]

Don't we also need to ensure that the new event listeners are removed on "blur" for the mainContainer, and also reset this._isScrolling = false; then?

Yes it's possible, but you've to alt+tab during scrolling: it's very unlikely to happen... That said I'll fix that.

[moz-tools-bot]

From: Bot.io (Linux m4)

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[moz-tools-bot]

From: Bot.io (Linux m4)

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Success

Full output at http://54.241.84.105:8877/5d021967e95770e/output.txt

Total script time: 1.20 mins

Published

• Viewer: http://54.241.84.105:8877/5d021967e95770e/web/viewer.html
• Viewer (legacy): http://54.241.84.105:8877/5d021967e95770e/legacy/web/viewer.html

[Snuffleupagus]

Having tested the preview there's unfortunately cases where the "scrollend" event doesn't seem to fire as expected, or possibly there's an extra "scroll"-event at the end of scrolling, which means that mouse-wheel zooming breaks.

• Case 1:

  1. Click on the scroll-wheel and move the mouse up or down, to trigger scrolling.
  2. Click again on the scroll-wheel, to stop scrolling.
  3. Hold Ctrl and scroll the mouse-wheel to zoom.

• Case 2:

  1. Hold the scroll-wheel and move the mouse up or down, to trigger scrolling.
  2. Release the scroll-wheel, to stop scrolling.
  3. Hold Ctrl and scroll the mouse-wheel to zoom.

[calixteman]

It's a Firefox bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1881974
That said I found a workaround.

[Snuffleupagus]

It's a Firefox bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1881974

Nice find, thanks for filing that!

That said I found a workaround.

Should we perhaps wait a couple of days, to see if there's a quick fix for the upstream bug, before landing a work-around?

[calixteman]

Should we perhaps wait a couple of days, to see if there's a quick fix for the upstream bug, before landing a work-around?

This bug is very likely there for a long time (I tested with a 2020 version) so I'm pretty sure that we won't have a fix soon (except maybe if it's easy enough to fix). So I think we should merge this PR (if everything is ok of course) and we can remove the workaround once it's fixed.

[moz-tools-bot]

From: Bot.io (Linux m4)

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From: Bot.io (Linux m4)

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Success

Full output at http://54.241.84.105:8877/b6d7c722fb8f775/output.txt

Total script time: 1.33 mins

Published

• Viewer: http://54.241.84.105:8877/b6d7c722fb8f775/web/viewer.html
• Viewer (legacy): http://54.241.84.105:8877/b6d7c722fb8f775/legacy/web/viewer.html

[Snuffleupagus]

r=me, now it seems to work in all cases I've tested; thank you!