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LibWeb: Respect scroll position set by script during page load

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When setting scroll position during page load we need to consider
whether we actually have a fragment to scroll to. A script may already
have run at that point and may already have set a scroll position.

If there is an actual fragment to scroll to, it is fine to scroll to
that fragment, since it should take precedence. If we don't have a
fragment however, we should not unnecessarily overwrite the scroll
position set by the script back to (0, 0).

Since this problem is caused by a spec bug, I have tested the behavior
in the three major browsers engines. Unfortunately they do not agree
fully with each other. If there is no fragment at all (e.g. `foo.html`),
all browsers will respect the scroll position set by the script. If
there is a fragment (e.g. `foo.html#bar`), all browsers will set the
scroll position to the fragment element and ignore the one set by
script. However, when the fragment is empty (e.g. `foo.html#`), then
Blink and WebKit will set scroll position to the fragment, while Gecko
will set scroll position from script. Since all of this is ad-hoc
behavior anyway, I simply implemented the Blink/WebKit behavior because
of the majority vote for now.

This fixes a regression introduced in 51102254b5.
This commit is contained in:
InvalidUsernameException 2025-03-07 22:04:36 +01:00 • committed by Alexander Kalenik
commit 0e1eb4d4a7
Notes: github-actions[bot] 2025-03-10 16:15:16 +00:00
3 changed files with 22 additions and 2 deletions
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12
Tests/LibWeb/Text/input/scroll-during-page-load.html Normal file
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<!DOCTYPE html>
<script src="include.js"></script>
<style>
div { height: 200vh; }
</style>
<div></div>
<script>
window.scrollTo(0, 200);
test(() => {
println(`Scroll position: ${window.scrollY}px`);
});
</script>