[UnifiedPDF][iOS] Relative scroll position is not preserved when resizing web view

[pxlcoder]

4312171

[UnifiedPDF][iOS] Relative scroll position is not preserved when resizing web view
https://bugs.webkit.org/show_bug.cgi?id=293906
rdar://152160616

Reviewed by Richard Robinson, Wenson Hsieh, and Tim Horton.

When the web view is resized, the fitting scale of the PDF changes, resulting
in an individual page becoming taller or shorter, and an overall change to the
content size. However, the scroll view's content offset remains unchanged,
resulting in a "jump" to another page when the web view is resized.

Fix by computing a new content offset when the fitting scale changes. Since the
fitting scale is not actually exposed to the UI process, simply take the ratio
of new content height to the old content height.

* Source/WebKit/UIProcess/API/ios/WKWebViewIOS.mm:
(-[WKWebView _updateScrollViewForTransaction:]):

Repurpose logic that pins the scroll position for traditional web content to
also support pinning the PDF scroll position.

Note that the added logic is only correct for PDFs that display vertically.
This is fine since WebKit does not support horizontal PDF display.

* Tools/TestWebKitAPI/Tests/WebKitCocoa/UnifiedPDFTests.mm:
(TestWebKitAPI::UNIFIED_PDF_TEST(KeepRelativeScrollPositionAfterAnimatedResize)):
(TestWebKitAPI::UNIFIED_PDF_TEST(ScrollOffsetResetWhenChangingPDF)):

Canonical link: https://commits.webkit.org/295715@main

44f36cb

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2	⏳ 🧪 win-tests
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🛠 wpe-cairo
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 vision	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	✅ 🧪 api-gtk
	✅ 🧪 vision-wk2	✅ 🧪 mac-intel-wk2	✅ 🛠 playstation
✅ 🛠 🧪 unsafe-merge	✅ 🛠 tv	✅ 🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash e5e8338)

Details

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	🧪 wpe-wk2	⏳ 🧪 win-tests
✅ 🧪 webkitperl	🧪 ios-wk2	🧪 api-mac	🧪 api-wpe
	⏳ 🧪 ios-wk2-wpt	🧪 mac-wk1	✅ 🛠 wpe-cairo
	🧪 api-ios	🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 vision	⏳ 🧪 mac-AS-debug-wk2	⏳ 🧪 gtk-wk2
	✅ 🛠 vision-sim	🧪 mac-wk2-stress	🧪 api-gtk
	🧪 vision-wk2	🧪 mac-intel-wk2	✅ 🛠 playstation
	✅ 🛠 tv	🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[hortont424]

This is fine since iOS WebKit does not support horizontal PDF display.

Neither does Mac! I think this is just an invented feature :P

[rr-codes]

should the oldContentSize.height > 0 just be oldContentSize.height because of the style rule not to compare to 0?

[whsieh]

The "compare to 0" style rule applies to equality checks

[pxlcoder]

Technically it would be fine since content size shouldn't be negative, but I also don't see a strong reason to change it, so keeping as-is.

[webkit-early-warning-system]

EWS run on current version of this PR (hash 44f36cb)

Details

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2	⏳ 🧪 win-tests
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🛠 wpe-cairo
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 vision	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	✅ 🧪 api-gtk
	✅ 🧪 vision-wk2	✅ 🧪 mac-intel-wk2	✅ 🛠 playstation
✅ 🛠 🧪 unsafe-merge	✅ 🛠 tv	✅ 🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-commit-queue]

Committed 295715@main (4312171): https://commits.webkit.org/295715@main

Reviewed commits have been landed. Closing PR #46211 and removing active labels.