From 345909c009679f5edf10cefca78089b4d1fe7eba Mon Sep 17 00:00:00 2001
From: Nico Weber <thakis@chromium.org>
Date: Fri, 22 Jan 2021 18:56:01 -0500
Subject: [PATCH] WindowServer: Don't crash on wallpapers smaller than the
 desktop with fill mode 'simple'

blit() calls draw_scaled_bitmap() behind the scenes in scaled contexts,
and that doesn't  like src_rect to be outside of the source bitmap's
bounds. Implicitly clip with the source rect, like the non-scaled
codepath already does.

Fixes #5017 even more.
---
 Userland/Demos/LibGfxScaleDemo/main.cpp       |  2 ++
 Userland/Libraries/LibGfx/Painter.cpp         | 10 +++++-----
 Userland/Services/WindowServer/Compositor.cpp |  2 +-
 3 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/Userland/Demos/LibGfxScaleDemo/main.cpp b/Userland/Demos/LibGfxScaleDemo/main.cpp
index ac11a2fa15..7352420d31 100644
--- a/Userland/Demos/LibGfxScaleDemo/main.cpp
+++ b/Userland/Demos/LibGfxScaleDemo/main.cpp
@@ -104,6 +104,8 @@ void Canvas::draw(Gfx::Painter& painter)
     painter.draw_scaled_bitmap({ 202, 39, 80, 40 }, *buggie, Gfx::IntRect { 2, 30, 62, 20 });
 
     painter.draw_tiled_bitmap({ 25, 60, WIDTH - 50, 40 }, *buggie);
+
+    painter.blit({ 25, 101 }, *buggie, { 2, 30, 3 * buggie->width(), 20 });
 }
 
 int main(int argc, char** argv)
diff --git a/Userland/Libraries/LibGfx/Painter.cpp b/Userland/Libraries/LibGfx/Painter.cpp
index bab4937eca..5368154933 100644
--- a/Userland/Libraries/LibGfx/Painter.cpp
+++ b/Userland/Libraries/LibGfx/Painter.cpp
@@ -660,11 +660,11 @@ void Painter::blit_offset(const IntPoint& a_position, const Gfx::Bitmap& source,
 
 void Painter::blit_with_alpha(const IntPoint& position, const Gfx::Bitmap& source, const IntRect& a_src_rect)
 {
+    auto safe_src_rect = a_src_rect.intersected(source.rect());
     if (scale() != source.scale())
-        return draw_scaled_bitmap({ position, a_src_rect.size() }, source, a_src_rect);
+        return draw_scaled_bitmap({ position, safe_src_rect.size() }, source, safe_src_rect);
 
     ASSERT(source.has_alpha_channel());
-    IntRect safe_src_rect = a_src_rect.intersected(source.rect());
     auto dst_rect = IntRect(position, safe_src_rect.size()).translated(translation());
 
     auto clipped_rect = dst_rect.intersected(clip_rect());
@@ -709,13 +709,13 @@ void Painter::blit(const IntPoint& position, const Gfx::Bitmap& source, const In
         return blit_with_opacity(position, source, a_src_rect, opacity);
     if (source.has_alpha_channel())
         return blit_with_alpha(position, source, a_src_rect);
+
+    auto safe_src_rect = a_src_rect.intersected(source.rect());
     if (scale() != source.scale())
-        return draw_scaled_bitmap({ position, a_src_rect.size() }, source, a_src_rect, opacity);
+        return draw_scaled_bitmap({ position, safe_src_rect.size() }, source, safe_src_rect, opacity);
 
     // If we get here, the Painter might have a scale factor, but the source bitmap has the same scale factor.
     // We need to transform from logical to physical coordinates, but we can just copy pixels without resampling.
-    auto safe_src_rect = a_src_rect.intersected(source.rect());
-    ASSERT(source.rect().contains(safe_src_rect));
     auto dst_rect = IntRect(position, safe_src_rect.size()).translated(translation());
     auto clipped_rect = dst_rect.intersected(clip_rect());
     if (clipped_rect.is_empty())
diff --git a/Userland/Services/WindowServer/Compositor.cpp b/Userland/Services/WindowServer/Compositor.cpp
index 9d2d025e02..38156ac218 100644
--- a/Userland/Services/WindowServer/Compositor.cpp
+++ b/Userland/Services/WindowServer/Compositor.cpp
@@ -246,7 +246,7 @@ void Compositor::compose()
     auto temp_painter = *m_temp_painter;
 
     auto paint_wallpaper = [&](Gfx::Painter& painter, const Gfx::IntRect& rect) {
-        // FIXME: If the wallpaper is opaque, no need to fill with color!
+        // FIXME: If the wallpaper is opaque and covers the whole rect, no need to fill with color!
         painter.fill_rect(rect, background_color);
         if (m_wallpaper) {
             if (m_wallpaper_mode == WallpaperMode::Simple) {