LibWeb: Implement VisualViewport aware viewport scrolling

Implements spec algorithm for viewport scrolling that first checks if
it's possible to use delta to move the visual viewport before falling
back to scrolling the layout viewport. This is a part of pinch-to-zoom
support.

Libraries/LibWeb/HTML/Navigable.cpp

@@ -7,6 +7,7 @@
  */
 
 #include <LibWeb/CSS/SystemColor.h>
+#include <LibWeb/CSS/VisualViewport.h>
 #include <LibWeb/ContentSecurityPolicy/BlockingAlgorithms.h>
 #include <LibWeb/ContentSecurityPolicy/Directives/DirectiveOperations.h>
 #include <LibWeb/ContentSecurityPolicy/PolicyList.h>
@@ -2710,4 +2711,62 @@ RefPtr<Gfx::SkiaBackendContext> Navigable::skia_backend_context() const
     return m_skia_backend_context;
 }
 
+// https://drafts.csswg.org/cssom-view/#viewport-perform-a-scroll
+void Navigable::scroll_viewport_by_delta(CSSPixelPoint delta)
+{
+    // 1. Let doc be the viewport’s associated Document.
+    auto doc = active_document();
+
+    // 2. Let vv be the VisualViewport whose associated document is doc.
+    auto vv = doc->visual_viewport();
+
+    CSSPixelRect viewport_rect = { m_viewport_scroll_offset, m_viewport_size };
+
+    // 3. Let maxX be the difference between viewport’s scrolling box’s width and the value of vv’s width attribute.
+    auto max_x = viewport_rect.width().to_double() - vv->width();
+
+    // 4. Let maxY be the difference between viewport’s scrolling box’s height and the value of vv’s height attribute.
+    auto max_y = viewport_rect.height().to_double() - vv->height();
+
+    // 5. Let dx be the horizontal component of position - the value vv’s pageLeft attribute
+    // NOTE: Function accepts delta so we use that directly.
+    auto dx = delta.x().to_double();
+
+    // 6. Let dy be the vertical component of position - the value of vv’s pageTop attribute
+    // NOTE: Function accepts delta so we use that directly.
+    auto dy = delta.y().to_double();
+
+    // 7. Let visual x be the value of vv’s offsetLeft attribute.
+    auto visual_x = vv->offset_left();
+
+    // 8. Let visual y be the value of vv’s offsetTop attribute.
+    auto visual_y = vv->offset_top();
+
+    // 9. Let visual dx be min(maxX, max(0, visual x + dx)) - visual x.
+    auto visual_dx = min(max_x, max(0.0, visual_x + dx)) - visual_x;
+
+    // 10. Let visual dy be min(maxY, max(0, visual y + dy)) - visual y.
+    auto visual_dy = min(max_y, max(0.0, visual_y + dy)) - visual_y;
+
+    // 11. Let layout dx be dx - visual dx
+    auto layout_dx = dx - visual_dx;
+
+    // 12. Let layout dy be dy - visual dy
+    auto layout_dy = dy - visual_dy;
+
+    // 13. Let element be doc’s root element if there is one, null otherwise.
+
+    // 14. Perform a scroll of the viewport’s scrolling box to its current scroll position + (layout dx, layout dy) with element as the associated element, and behavior as the scroll behavior.
+    auto scrolling_area = doc->paintable_box()->scrollable_overflow_rect()->to_type<float>();
+    auto new_viewport_scroll_offset = m_viewport_scroll_offset.to_type<double>() + Gfx::Point(layout_dx, layout_dy);
+    // NOTE: Clamp to the scrolling area.
+    new_viewport_scroll_offset.set_x(max(0.0, min(new_viewport_scroll_offset.x(), scrolling_area.width() - viewport_size().width().to_double())));
+    new_viewport_scroll_offset.set_y(max(0.0, min(new_viewport_scroll_offset.y(), scrolling_area.height() - viewport_size().height().to_double())));
+    perform_scroll_of_viewport(new_viewport_scroll_offset.to_type<CSSPixels>());
+
+    // 15. Perform a scroll of vv’s scrolling box to its current scroll position + (visual dx, visual dy) with element as the associated element, and behavior as the scroll behavior.
+    vv->scroll_by({ visual_dx, visual_dy });
+    doc->set_needs_display(InvalidateDisplayList::No);
+}
+
 }