WindowServer: Add "Natural scrolling" support

Also commonly referred to as "reverse scrolling" or "inverted
scrolling".

Base/etc/WindowServer.ini

@@ -23,6 +23,7 @@ AccelerationFactor=1.0
 ScrollStepSize=4
 CursorTheme=Default
 ButtonsSwitched=false
+NaturalScroll=false
 
 [Graphics]
 OverlayRectShadow=/res/graphics/overlay-rect-shadow.png

Userland/Services/WindowServer/ConnectionFromClient.cpp

@@ -1110,6 +1110,16 @@ Messages::WindowServer::GetButtonsSwitchedResponse ConnectionFromClient::get_but
     return WindowManager::the().get_buttons_switched();
 }
 
+void ConnectionFromClient::set_natural_scroll(bool inverted)
+{
+    WindowManager::the().set_natural_scroll(inverted);
+}
+
+Messages::WindowServer::IsNaturalScrollResponse ConnectionFromClient::is_natural_scroll()
+{
+    return WindowManager::the().is_natural_scroll();
+}
+
 void ConnectionFromClient::set_unresponsive(bool unresponsive)
 {
     if (m_unresponsive == unresponsive)

Userland/Services/WindowServer/ConnectionFromClient.h

@@ -178,6 +178,8 @@ private:
     virtual Messages::WindowServer::GetDoubleClickSpeedResponse get_double_click_speed() override;
     virtual void set_buttons_switched(bool) override;
     virtual Messages::WindowServer::GetButtonsSwitchedResponse get_buttons_switched() override;
+    virtual void set_natural_scroll(bool) override;
+    virtual Messages::WindowServer::IsNaturalScrollResponse is_natural_scroll() override;
     virtual void set_window_modified(i32, bool) override;
     virtual Messages::WindowServer::IsWindowModifiedResponse is_window_modified(i32) override;
     virtual Messages::WindowServer::GetDesktopDisplayScaleResponse get_desktop_display_scale(u32) override;

Userland/Services/WindowServer/EventLoop.cpp

@@ -70,10 +70,15 @@ void EventLoop::drain_mouse()
             state.x = packet.x;
             state.y = packet.y;
         }
-        state.z += packet.z;
         state.w += packet.w;
         state_is_sent = false;
 
+        // Invert scroll direction if checked in the settings.
+        if (WindowManager::the().is_natural_scroll())
+            state.z -= packet.z;
+        else
+            state.z += packet.z;
+
         if (packet.buttons != state.buttons) {
             state.buttons = packet.buttons;
             dbgln_if(WSMESSAGELOOP_DEBUG, "EventLoop: Mouse Button Event");

Userland/Services/WindowServer/WindowManager.cpp

@@ -83,6 +83,7 @@ void WindowManager::reload_config()
 
     m_double_click_speed = m_config->read_num_entry("Input", "DoubleClickSpeed", 250);
     m_buttons_switched = m_config->read_bool_entry("Mouse", "ButtonsSwitched", false);
+    m_natural_scroll = m_config->read_bool_entry("Mouse", "NaturalScroll", false);
     m_cursor_highlight_radius = m_config->read_num_entry("Mouse", "CursorHighlightRadius", 25);
     Color default_highlight_color = Color::NamedColor::Red;
     default_highlight_color.set_alpha(110);
@@ -305,6 +306,19 @@ bool WindowManager::get_buttons_switched() const
     return m_buttons_switched;
 }
 
+void WindowManager::set_natural_scroll(bool inverted)
+{
+    m_natural_scroll = inverted;
+    dbgln("Saving scroll inverted state {} to config file at {}", inverted, m_config->filename());
+    m_config->write_bool_entry("Mouse", "NaturalScroll", inverted);
+    sync_config_to_disk();
+}
+
+bool WindowManager::is_natural_scroll() const
+{
+    return m_natural_scroll;
+}
+
 WindowStack& WindowManager::window_stack_for_window(Window& window)
 {
     if (is_stationary_window_type(window.type()))

Userland/Services/WindowServer/WindowManager.h

@@ -153,6 +153,8 @@ public:
     int double_click_speed() const;
     void set_buttons_switched(bool);
     bool get_buttons_switched() const;
+    void set_natural_scroll(bool);
+    bool is_natural_scroll() const;
 
     void set_active_window(Window*);
     void set_hovered_button(Button*);
@@ -433,6 +435,7 @@ private:
     int m_max_distance_for_double_click { 4 };
     bool m_previous_event_was_super_keydown { false };
     bool m_buttons_switched { false };
+    bool m_natural_scroll { false };
     bool m_theme_overridden { false };
 
     WeakPtr<Window> m_hovered_window;

Userland/Services/WindowServer/WindowServer.ipc

@@ -176,6 +176,9 @@ endpoint WindowServer
     set_buttons_switched(bool switched) =|
     get_buttons_switched() => (bool switched)
 
+    set_natural_scroll(bool inverted) =|
+    is_natural_scroll() => (bool inverted)
+
     get_desktop_display_scale(u32 screen_index) => (int desktop_display_scale)
 
     set_flash_flush(bool enabled) =|