LibWeb: Move adjustmust of scrollbar offset into compute_scrollbar_data

Currently, compute_scrollbar_data does not adjust the position of the
scrollbar thumb based on the actual scroll offset. This is because we
perform this offset in most cases inside the display list executor, in
order to allow us to avoid recomputing the display list.

However, there are cases where we do want the thumb rect with an offset
inside PaintableBox. We currently use scroll_thumb_rect to perform that
computation.

In an upcoming patch, we will need both this offset thumb rect and the
scrollbar gutter rect. So this patch moves the computation of the offset
to compute_scrollbar_data, performed behind an optional parameter.

Libraries/LibWeb/Painting/PaintableBox.cpp

@@ -310,9 +310,9 @@ Optional<CSSPixelRect> PaintableBox::get_clip_rect() const
 
 bool PaintableBox::wants_mouse_events() const
 {
-    if (scroll_thumb_rect(ScrollDirection::Vertical).has_value())
+    if (compute_scrollbar_data(ScrollDirection::Vertical).has_value())
         return true;
-    if (scroll_thumb_rect(ScrollDirection::Horizontal).has_value())
+    if (compute_scrollbar_data(ScrollDirection::Horizontal).has_value())
         return true;
     return false;
 }
@@ -361,25 +361,7 @@ bool PaintableBox::could_be_scrolled_by_wheel_event() const
 static constexpr CSSPixels SCROLLBAR_THUMB_NORMAL_THICKNESS = 5;
 static constexpr CSSPixels SCROLLBAR_THUMB_WIDENED_THICKNESS = 10;
 
-Optional<CSSPixelRect> PaintableBox::scroll_thumb_rect(ScrollDirection direction) const
+Optional<PaintableBox::ScrollbarData> PaintableBox::compute_scrollbar_data(ScrollDirection direction, AdjustThumbRectForScrollOffset adjust_thumb_rect_for_scroll_offset) const
-{
-    auto maybe_scrollbar_data = compute_scrollbar_data(direction);
-    if (!maybe_scrollbar_data.has_value())
-        return {};
-
-    auto scroll_offset = direction == ScrollDirection::Horizontal ? -own_scroll_frame_offset().x() : -own_scroll_frame_offset().y();
-    auto thumb_offset = scroll_offset * maybe_scrollbar_data->scroll_length;
-
-    CSSPixelRect thumb_rect = maybe_scrollbar_data->thumb_rect;
-    if (direction == ScrollDirection::Horizontal) {
-        thumb_rect.translate_by(thumb_offset, 0);
-    } else {
-        thumb_rect.translate_by(0, thumb_offset);
-    }
-    return thumb_rect;
-}
-
-Optional<PaintableBox::ScrollbarData> PaintableBox::compute_scrollbar_data(ScrollDirection direction) const
 {
     bool is_horizontal = direction == ScrollDirection::Horizontal;
     bool display_scrollbar = could_be_scrolled_by_wheel_event(direction);
@@ -429,6 +411,16 @@ Optional<PaintableBox::ScrollbarData> PaintableBox::compute_scrollbar_data(Scrol
         scrollbar_data.thumb_rect = { padding_rect.right() - thickness, padding_rect.top(), thickness, thumb_length };
     }
 
+    if (adjust_thumb_rect_for_scroll_offset == AdjustThumbRectForScrollOffset::Yes) {
+        auto scroll_offset = is_horizontal ? -own_scroll_frame_offset().x() : -own_scroll_frame_offset().y();
+        auto thumb_offset = scroll_offset * scrollbar_data.scroll_length;
+
+        if (is_horizontal)
+            scrollbar_data.thumb_rect.translate_by(thumb_offset, 0);
+        else
+            scrollbar_data.thumb_rect.translate_by(0, thumb_offset);
+    }
+
     return scrollbar_data;
 }
 
@@ -920,17 +912,19 @@ void PaintableWithLines::paint(PaintContext& context, PaintPhase phase) const
 
 Paintable::DispatchEventOfSameName PaintableBox::handle_mousedown(Badge<EventHandler>, CSSPixelPoint position, unsigned, unsigned)
 {
-    auto vertical_scroll_thumb_rect = scroll_thumb_rect(ScrollDirection::Vertical);
+    auto vertical_scroll_data = compute_scrollbar_data(ScrollDirection::Vertical, AdjustThumbRectForScrollOffset::Yes);
-    auto horizontal_scroll_thumb_rect = scroll_thumb_rect(ScrollDirection::Horizontal);
+    auto horizontal_scroll_data = compute_scrollbar_data(ScrollDirection::Horizontal, AdjustThumbRectForScrollOffset::Yes);
-    if (vertical_scroll_thumb_rect.has_value() && vertical_scroll_thumb_rect.value().contains(position)) {
+
+    if (vertical_scroll_data.has_value() && vertical_scroll_data->thumb_rect.contains(position)) {
         m_last_mouse_tracking_position = position;
         m_scroll_thumb_dragging_direction = ScrollDirection::Vertical;
         const_cast<HTML::Navigable&>(*navigable()).event_handler().set_mouse_event_tracking_paintable(this);
-    } else if (horizontal_scroll_thumb_rect.has_value() && horizontal_scroll_thumb_rect.value().contains(position)) {
+    } else if (horizontal_scroll_data.has_value() && horizontal_scroll_data->thumb_rect.contains(position)) {
         m_last_mouse_tracking_position = position;
         m_scroll_thumb_dragging_direction = ScrollDirection::Horizontal;
         const_cast<HTML::Navigable&>(*navigable()).event_handler().set_mouse_event_tracking_paintable(this);
     }
+
     return Paintable::DispatchEventOfSameName::Yes;
 }
 

Libraries/LibWeb/Painting/PaintableBox.h

@@ -266,8 +266,11 @@ protected:
         Horizontal,
         Vertical,
     };
-    Optional<ScrollbarData> compute_scrollbar_data(ScrollDirection) const;
+    enum class AdjustThumbRectForScrollOffset {
-    [[nodiscard]] Optional<CSSPixelRect> scroll_thumb_rect(ScrollDirection) const;
+        No,
+        Yes,
+    };
+    Optional<ScrollbarData> compute_scrollbar_data(ScrollDirection, AdjustThumbRectForScrollOffset = AdjustThumbRectForScrollOffset::No) const;
     [[nodiscard]] bool could_be_scrolled_by_wheel_event(ScrollDirection) const;
 
     TraversalDecision hit_test_scrollbars(CSSPixelPoint position, Function<TraversalDecision(HitTestResult)> const& callback) const;