Auto merge of #17385 - pyfisch:better-scroll, r=mrobinson

Fix several bugs related to scrolling

* scrollLeft/scrollTop returned values of parent or even document root
   Only the scroll of the node itself is returned. Otherwise 0.0.
* Scrolling via script had set viewport.
   This resulted in other nodes appearing scrolled.
   Now scroll_offsets are updated with correct node id.

These bugs caused other odd behavior like both body and
document.documentElement being scrolled or the view for scrolled
elements jumping.

Also try scrolling this [example page](https://pyfisch.org/stuff/scrolltest.html) in servo with and without this change.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #17342 (github issue number if applicable).

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- [x] There are tests for these changes (partially)

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1 files changed, 1 insertions, 0 deletions

diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index d603d898f5c..c97d352872c 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -703,6 +703,7 @@ impl Document {
             // Step 3
             let global_scope = self.window.upcast::<GlobalScope>();
             let webrender_pipeline_id = global_scope.pipeline_id().to_webrender();
+            self.window.update_viewport_for_scroll(x, y);
             self.window.perform_a_scroll(x,
                                          y,
                                          ClipId::root_scroll_node(webrender_pipeline_id),