LibGUI: Add visual clue to Scrollbar gutter

This adds a visual clue to scrolling by clicking on the Scrollbar
gutter. This gives the user a hint that scrolling will continue in the
direction of the darkened gutter, until the mouse is released.
It is inspired by very similar behavior on old windows.

Userland/Libraries/LibGUI/Scrollbar.cpp

@@ -174,6 +174,30 @@ void Scrollbar::paint_event(PaintEvent& event)
         hovered_component_for_painting = Component::None;
 
     painter.fill_rect_with_dither_pattern(rect(), palette().button().lightened(1.3f), palette().button());
+    if (gutter_click_state != GutterClickState::NotPressed && has_scrubber() && hovered_component_for_painting == Component::Gutter) {
+        VERIFY(!scrubber_rect().is_null());
+        Gfx::IntRect rect_to_fill = rect();
+        if (orientation() == Orientation::Vertical) {
+            if (gutter_click_state == GutterClickState::BeforeScrubber) {
+                rect_to_fill.set_top(decrement_button_rect().bottom());
+                rect_to_fill.set_bottom(scrubber_rect().top());
+            } else {
+                VERIFY(gutter_click_state == GutterClickState::AfterScrubber);
+                rect_to_fill.set_top(scrubber_rect().bottom());
+                rect_to_fill.set_bottom(increment_button_rect().top());
+            }
+        } else {
+            if (gutter_click_state == GutterClickState::BeforeScrubber) {
+                rect_to_fill.set_left(decrement_button_rect().right());
+                rect_to_fill.set_right(scrubber_rect().left());
+            } else {
+                VERIFY(gutter_click_state == GutterClickState::AfterScrubber);
+                rect_to_fill.set_left(scrubber_rect().right());
+                rect_to_fill.set_right(increment_button_rect().left());
+            }
+        }
+        painter.fill_rect_with_dither_pattern(rect_to_fill, palette().button(), palette().button().lightened(0.77f));
+    }
 
     bool decrement_pressed = (m_pressed_component == Component::DecrementButton) && (m_pressed_component == m_hovered_component);
     bool increment_pressed = (m_pressed_component == Component::IncrementButton) && (m_pressed_component == m_hovered_component);
@@ -213,9 +237,15 @@ void Scrollbar::on_automatic_scrolling_timer_fired()
     }
     if (m_pressed_component == Component::Gutter && component_at_position(m_last_mouse_position) == Component::Gutter) {
         scroll_by_page(m_last_mouse_position);
-        m_hovered_component = component_at_position(m_last_mouse_position);
+        if (m_hovered_component != component_at_position(m_last_mouse_position)) {
+            m_hovered_component = component_at_position(m_last_mouse_position);
+            if (m_hovered_component != Component::Gutter)
+                gutter_click_state = GutterClickState::NotPressed;
+            update();
+        }
         return;
     }
+    gutter_click_state = GutterClickState::NotPressed;
 }
 
 void Scrollbar::mousedown_event(MouseEvent& event)
@@ -286,6 +316,7 @@ void Scrollbar::set_automatic_scrolling_active(bool active, Component pressed_co
         m_automatic_scrolling_timer->start();
     } else {
         m_automatic_scrolling_timer->stop();
+        gutter_click_state = GutterClickState::NotPressed;
     }
 }
 
@@ -296,10 +327,13 @@ void Scrollbar::scroll_by_page(const Gfx::IntPoint& click_position)
     float rel_scrubber_size = unclamped_scrubber_size() / available;
     float page_increment = range_size * rel_scrubber_size;
 
-    if (click_position.primary_offset_for_orientation(orientation()) < scrubber_rect().primary_offset_for_orientation(orientation()))
+    if (click_position.primary_offset_for_orientation(orientation()) < scrubber_rect().primary_offset_for_orientation(orientation())) {
+        gutter_click_state = GutterClickState::BeforeScrubber;
         set_value(value() - page_increment);
-    else
+    } else {
+        gutter_click_state = GutterClickState::AfterScrubber;
         set_value(value() + page_increment);
+    }
 }
 
 void Scrollbar::scroll_to_position(const Gfx::IntPoint& click_position)

Userland/Libraries/LibGUI/Scrollbar.h

@@ -41,6 +41,13 @@ protected:
     virtual void change_event(Event&) override;
 
 private:
+    enum class GutterClickState {
+        NotPressed,
+        BeforeScrubber,
+        AfterScrubber,
+    } gutter_click_state
+        = GutterClickState::NotPressed;
+
     int default_button_size() const { return 16; }
     int button_size() const { return length(orientation()) <= (default_button_size() * 2) ? length(orientation()) / 2 : default_button_size(); }
     int button_width() const { return orientation() == Orientation::Vertical ? width() : button_size(); }