From 272a41d02d6351e52e08a262818170da58147d0b Mon Sep 17 00:00:00 2001
From: Andreas Kling <awesomekling@gmail.com>
Date: Sat, 9 Mar 2019 21:38:28 +0100
Subject: [PATCH] WindowServer: Skip composing windows where entire dirty rect
 is covered.

If another window is currently in front of us and is fully opaque,
we can just skip to the next window. This avoids a whole lot of painting
with overlapping windows. :^)
---
 WindowServer/WSWindowManager.cpp | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/WindowServer/WSWindowManager.cpp b/WindowServer/WSWindowManager.cpp
index 7dcb28f889..1a0fed5faf 100644
--- a/WindowServer/WSWindowManager.cpp
+++ b/WindowServer/WSWindowManager.cpp
@@ -871,6 +871,20 @@ void WSWindowManager::compose()
         return false;
     };
 
+    auto any_opaque_window_above_this_one_contains_rect = [this] (const WSWindow& a_window, const Rect& r) {
+        for (auto* window = a_window.next(); window; window = window->next()) {
+            if (!window->is_visible())
+                continue;
+            if (window->opacity() < 1.0f)
+                continue;
+            if (window->has_alpha_channel())
+                continue;
+            if (outer_window_rect(*window).contains(r))
+                return true;
+        }
+        return false;
+    };
+
     auto any_dirty_rect_intersects_window = [&dirty_rects] (const WSWindow& window) {
         auto window_rect = outer_window_rect(window);
         for (auto& dirty_rect : dirty_rects.rects()) {
@@ -898,6 +912,8 @@ void WSWindowManager::compose()
         PainterStateSaver saver(*m_back_painter);
         m_back_painter->set_clip_rect(outer_window_rect(window));
         for (auto& dirty_rect : dirty_rects.rects()) {
+            if (any_opaque_window_above_this_one_contains_rect(window, dirty_rect))
+                continue;
             PainterStateSaver saver(*m_back_painter);
             m_back_painter->set_clip_rect(dirty_rect);
             paint_window_frame(window);