From dd0833107f4e60d84ec2353d43a8e9b4fb073eaa Mon Sep 17 00:00:00 2001
From: Tom <tomut@yahoo.com>
Date: Wed, 10 Feb 2021 23:08:47 -0700
Subject: [PATCH] WindowServer: Fix clearing area not covered by backing bitmap

We only cleared the area not covered by the backing bitmap if a
rendering rectangle intersected with the backing bitmap. But because
we are potentially calling the render function many times we need
to always clear the area not covered by the backing bitmap, whether
it intersects or not.

Fixes #5291
---
 Userland/Services/WindowServer/Compositor.cpp | 51 ++++++++++---------
 1 file changed, 28 insertions(+), 23 deletions(-)

diff --git a/Userland/Services/WindowServer/Compositor.cpp b/Userland/Services/WindowServer/Compositor.cpp
index f5b338061b..7de34140b6 100644
--- a/Userland/Services/WindowServer/Compositor.cpp
+++ b/Userland/Services/WindowServer/Compositor.cpp
@@ -275,7 +275,8 @@ void Compositor::compose()
         auto frame_rect = window.frame().render_rect();
         if (!frame_rect.intersects(ws.rect()))
             return IterationDecision::Continue;
-        auto frame_rects = frame_rect.shatter(window.rect());
+        auto window_rect = window.rect();
+        auto frame_rects = frame_rect.shatter(window_rect);
 
         dbgln_if(COMPOSE_DEBUG, "  window {} frame rect: {}", window.title(), frame_rect);
 
@@ -291,9 +292,15 @@ void Compositor::compose()
                 });
             }
 
+            auto clear_window_rect = [&](const Gfx::IntRect& clear_rect) {
+                auto fill_color = wm.palette().window();
+                if (!window.is_opaque())
+                    fill_color.set_alpha(255 * window.opacity());
+                painter.fill_rect(clear_rect, fill_color);
+            };
+
             if (!backing_store) {
-                if (window.is_opaque())
-                    painter.fill_rect(window.rect().intersected(rect), wm.palette().window());
+                clear_window_rect(window_rect.intersected(rect));
                 return;
             }
 
@@ -312,44 +319,42 @@ void Compositor::compose()
             case ResizeDirection::Right:
             case ResizeDirection::Down:
             case ResizeDirection::DownRight:
-                backing_rect.set_location(window.rect().location());
+                backing_rect.set_location(window_rect.location());
                 break;
             case ResizeDirection::Left:
             case ResizeDirection::Up:
             case ResizeDirection::UpLeft:
-                backing_rect.set_right_without_resize(window.rect().right());
-                backing_rect.set_bottom_without_resize(window.rect().bottom());
+                backing_rect.set_right_without_resize(window_rect.right());
+                backing_rect.set_bottom_without_resize(window_rect.bottom());
                 break;
             case ResizeDirection::UpRight:
                 backing_rect.set_left(window.rect().left());
-                backing_rect.set_bottom_without_resize(window.rect().bottom());
+                backing_rect.set_bottom_without_resize(window_rect.bottom());
                 break;
             case ResizeDirection::DownLeft:
-                backing_rect.set_right_without_resize(window.rect().right());
-                backing_rect.set_top(window.rect().top());
+                backing_rect.set_right_without_resize(window_rect.right());
+                backing_rect.set_top(window_rect.top());
                 break;
             }
 
-            Gfx::IntRect dirty_rect_in_backing_coordinates = rect.intersected(window.rect())
+            Gfx::IntRect dirty_rect_in_backing_coordinates = rect.intersected(window_rect)
                                                                  .intersected(backing_rect)
                                                                  .translated(-backing_rect.location());
 
-            if (dirty_rect_in_backing_coordinates.is_empty())
-                return;
-            auto dst = backing_rect.location().translated(dirty_rect_in_backing_coordinates.location());
+            if (!dirty_rect_in_backing_coordinates.is_empty()) {
+                auto dst = backing_rect.location().translated(dirty_rect_in_backing_coordinates.location());
 
-            if (window.client() && window.client()->is_unresponsive()) {
-                painter.blit_filtered(dst, *backing_store, dirty_rect_in_backing_coordinates, [](Color src) {
-                    return src.to_grayscale().darkened(0.75f);
-                });
-            } else {
-                painter.blit(dst, *backing_store, dirty_rect_in_backing_coordinates, window.opacity());
+                if (window.client() && window.client()->is_unresponsive()) {
+                    painter.blit_filtered(dst, *backing_store, dirty_rect_in_backing_coordinates, [](Color src) {
+                        return src.to_grayscale().darkened(0.75f);
+                    });
+                } else {
+                    painter.blit(dst, *backing_store, dirty_rect_in_backing_coordinates, window.opacity());
+                }
             }
 
-            if (window.is_opaque()) {
-                for (auto background_rect : window.rect().shatter(backing_rect))
-                    painter.fill_rect(background_rect, wm.palette().window());
-            }
+            for (auto background_rect : window_rect.shatter(backing_rect))
+                clear_window_rect(background_rect);
         };
 
         auto& dirty_rects = window.dirty_rects();