LibWeb: Allow scrolling overflowed content with the mouse wheel :^)

This is rather crude, but you can now use the mouse wheel to scroll up
and down in block-level boxes with clipped overflowing content.
There's no limit to how far you can scroll in either direction, since
we don't yet track how much overflow there is. But it's a start. :^)

Userland/Libraries/LibWeb/Layout/BlockBox.cpp

@@ -68,6 +68,7 @@ void BlockBox::paint(PaintContext& context, PaintPhase phase)
         context.painter().save();
         // FIXME: Handle overflow-x and overflow-y being different values.
         context.painter().add_clip_rect(enclosing_int_rect(padded_rect()));
+        context.painter().translate(-m_scroll_offset.to_type<int>());
     }
 
     for (auto& line_box : m_line_boxes) {
@@ -141,4 +142,19 @@ void BlockBox::split_into_lines(InlineFormattingContext& context, LayoutMode lay
     line_box->add_fragment(*this, 0, 0, border_box_width(), height());
 }
 
+void BlockBox::set_scroll_offset(const Gfx::FloatPoint& offset)
+{
+    if (m_scroll_offset == offset)
+        return;
+    m_scroll_offset = offset;
+    set_needs_display();
+}
+
+void BlockBox::handle_mousewheel(Badge<EventHandler>, const Gfx::IntPoint&, unsigned int, unsigned int, int wheel_delta)
+{
+    auto new_offset = m_scroll_offset;
+    new_offset.move_by(0, wheel_delta);
+    set_scroll_offset(new_offset);
+}
+
 }

Userland/Libraries/LibWeb/Layout/BlockBox.h

@@ -53,10 +53,17 @@ public:
 
     virtual void split_into_lines(InlineFormattingContext&, LayoutMode) override;
 
+    const Gfx::FloatPoint& scroll_offset() const { return m_scroll_offset; }
+    void set_scroll_offset(const Gfx::FloatPoint&);
+
 private:
     virtual bool is_block_box() const final { return true; }
+    virtual bool wants_mouse_events() const override { return true; }
+    virtual void handle_mousewheel(Badge<EventHandler>, const Gfx::IntPoint&, unsigned buttons, unsigned modifiers, int wheel_delta) override;
 
     bool should_clip_overflow() const;
+
+    Gfx::FloatPoint m_scroll_offset;
 };
 
 template<>

Userland/Libraries/LibWeb/Layout/Node.cpp

@@ -318,6 +318,15 @@ void Node::handle_mousemove(Badge<EventHandler>, const Gfx::IntPoint&, unsigned,
 {
 }
 
+void Node::handle_mousewheel(Badge<EventHandler>, const Gfx::IntPoint&, unsigned, unsigned, int wheel_delta)
+{
+    if (auto* containing_block = this->containing_block()) {
+        auto new_offset = containing_block->scroll_offset();
+        new_offset.move_by(0, wheel_delta);
+        containing_block->set_scroll_offset(new_offset);
+    }
+}
+
 bool Node::is_root_element() const
 {
     if (is_anonymous())

Userland/Libraries/LibWeb/Layout/Node.h

@@ -109,6 +109,7 @@ public:
     virtual void handle_mousedown(Badge<EventHandler>, const Gfx::IntPoint&, unsigned button, unsigned modifiers);
     virtual void handle_mouseup(Badge<EventHandler>, const Gfx::IntPoint&, unsigned button, unsigned modifiers);
     virtual void handle_mousemove(Badge<EventHandler>, const Gfx::IntPoint&, unsigned buttons, unsigned modifiers);
+    virtual void handle_mousewheel(Badge<EventHandler>, const Gfx::IntPoint&, unsigned buttons, unsigned modifiers, int wheel_delta);
 
     virtual void before_children_paint(PaintContext&, PaintPhase) {};
     virtual void paint(PaintContext&, PaintPhase);

Userland/Libraries/LibWeb/Page/EventHandler.cpp

@@ -75,6 +75,17 @@ Layout::InitialContainingBlockBox* EventHandler::layout_root()
 
 bool EventHandler::handle_mousewheel(const Gfx::IntPoint& position, unsigned int buttons, unsigned int modifiers, int wheel_delta)
 {
+    if (!layout_root())
+        return false;
+
+    // FIXME: Support wheel events in subframes.
+
+    auto result = layout_root()->hit_test(position, Layout::HitTestType::Exact);
+    if (result.layout_node) {
+        result.layout_node->handle_mousewheel({}, position, buttons, modifiers, wheel_delta);
+        return true;
+    }
+
     return false;
 }