From 1fc887c5767cfced4df3af10d1f0d15685c1022f Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Tue, 11 Feb 2020 11:33:01 +0100
Subject: [PATCH] LibGUI: Only highlight grabbable area between a Splitter's
 children

We were previously cheating by setting the entire splitter's background
color to the hover highlight color. This looked goofy whenever there
were transparent widgets inside a splitter, since the highlighted color
would shine through when hovering.

This was especially noticeable in SystemMonitor, which now looks much
better. :^)
---
 Libraries/LibGUI/Splitter.cpp | 52 +++++++++++++++++++++++++++++++----
 Libraries/LibGUI/Splitter.h   |  6 ++++
 2 files changed, 52 insertions(+), 6 deletions(-)

diff --git a/Libraries/LibGUI/Splitter.cpp b/Libraries/LibGUI/Splitter.cpp
index 8783301267..5a7ca6dba8 100644
--- a/Libraries/LibGUI/Splitter.cpp
+++ b/Libraries/LibGUI/Splitter.cpp
@@ -24,10 +24,11 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <LibGfx/Palette.h>
 #include <LibGUI/BoxLayout.h>
+#include <LibGUI/Painter.h>
 #include <LibGUI/Splitter.h>
 #include <LibGUI/Window.h>
+#include <LibGfx/Palette.h>
 
 namespace GUI {
 
@@ -45,19 +46,32 @@ Splitter::~Splitter()
 {
 }
 
+void Splitter::paint_event(PaintEvent& event)
+{
+    Painter painter(*this);
+    painter.add_clip_rect(event.rect());
+    painter.fill_rect(m_grabbable_rect, palette().hover_highlight());
+}
+
+void Splitter::resize_event(ResizeEvent& event)
+{
+    Frame::resize_event(event);
+    m_grabbable_rect = {};
+}
+
 void Splitter::enter_event(Core::Event&)
 {
-    set_background_role(ColorRole::HoverHighlight);
     window()->set_override_cursor(m_orientation == Orientation::Horizontal ? StandardCursor::ResizeHorizontal : StandardCursor::ResizeVertical);
-    update();
 }
 
 void Splitter::leave_event(Core::Event&)
 {
-    set_background_role(ColorRole::Button);
     if (!m_resizing)
         window()->set_override_cursor(StandardCursor::None);
-    update();
+    if (!m_grabbable_rect.is_empty()) {
+        m_grabbable_rect = {};
+        update();
+    }
 }
 
 void Splitter::get_resize_candidates_at(const Gfx::Point& position, Widget*& first, Widget*& second)
@@ -94,10 +108,30 @@ void Splitter::mousedown_event(MouseEvent& event)
     m_resize_origin = event.position();
 }
 
+void Splitter::recompute_grabbable_rect(const Widget& first, const Widget& second)
+{
+    auto first_edge = first.relative_rect().primary_offset_for_orientation(m_orientation) + first.relative_rect().primary_size_for_orientation(m_orientation);
+    auto second_edge = second.relative_rect().primary_offset_for_orientation(m_orientation);
+    Gfx::Rect rect;
+    rect.set_primary_offset_for_orientation(m_orientation, first_edge);
+    rect.set_primary_size_for_orientation(m_orientation, second_edge - first_edge);
+    rect.set_secondary_offset_for_orientation(m_orientation, first.relative_rect().secondary_offset_for_orientation(m_orientation));
+    rect.set_secondary_size_for_orientation(m_orientation, first.relative_rect().secondary_size_for_orientation(m_orientation));
+    if (m_grabbable_rect != rect) {
+        m_grabbable_rect = rect;
+        update();
+    }
+}
+
 void Splitter::mousemove_event(MouseEvent& event)
 {
-    if (!m_resizing)
+    if (!m_resizing) {
+        Widget* first { nullptr };
+        Widget* second { nullptr };
+        get_resize_candidates_at(event.position(), first, second);
+        recompute_grabbable_rect(*first, *second);
         return;
+    }
     auto delta = event.position() - m_resize_origin;
     if (!m_first_resizee || !m_second_resizee) {
         // One or both of the resizees were deleted during an ongoing resize, screw this.
@@ -130,6 +164,12 @@ void Splitter::mousemove_event(MouseEvent& event)
     invalidate_layout();
 }
 
+void Splitter::did_layout()
+{
+    if (m_first_resizee && m_second_resizee)
+        recompute_grabbable_rect(*m_first_resizee, *m_second_resizee);
+}
+
 void Splitter::mouseup_event(MouseEvent& event)
 {
     if (event.button() != MouseButton::Left)
diff --git a/Libraries/LibGUI/Splitter.h b/Libraries/LibGUI/Splitter.h
index 4cf876648c..1cf946b608 100644
--- a/Libraries/LibGUI/Splitter.h
+++ b/Libraries/LibGUI/Splitter.h
@@ -38,13 +38,18 @@ public:
 protected:
     Splitter(Orientation, Widget* parent);
 
+    virtual void paint_event(PaintEvent&) override;
+    virtual void resize_event(ResizeEvent&) override;
     virtual void mousedown_event(MouseEvent&) override;
     virtual void mousemove_event(MouseEvent&) override;
     virtual void mouseup_event(MouseEvent&) override;
     virtual void enter_event(Core::Event&) override;
     virtual void leave_event(Core::Event&) override;
 
+    virtual void did_layout() override;
+
 private:
+    void recompute_grabbable_rect(const Widget&, const Widget&);
     void get_resize_candidates_at(const Gfx::Point&, Widget*&, Widget*&);
 
     Orientation m_orientation;
@@ -54,6 +59,7 @@ private:
     WeakPtr<Widget> m_second_resizee;
     Gfx::Size m_first_resizee_start_size;
     Gfx::Size m_second_resizee_start_size;
+    Gfx::Rect m_grabbable_rect;
 };
 
 class VerticalSplitter final : public Splitter {