From 92485ed222d7841bc8994ceddedb85a68420ab96 Mon Sep 17 00:00:00 2001
From: Gleb Mazovetskiy <glex.spb@gmail.com>
Date: Sun, 18 Apr 2021 23:55:42 +0100
Subject: [PATCH] DiabloUI: Support non-doublebuf HW surfaces

If using a hardware surface without double-buffering, perform
rendering in a single blit to avoid flickering and tearing.
---
 Source/DiabloUI/diabloui.cpp |  4 ++++
 Source/DiabloUI/diabloui.h   | 13 ++++++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/Source/DiabloUI/diabloui.cpp b/Source/DiabloUI/diabloui.cpp
index 6c52e2740..10f479471 100644
--- a/Source/DiabloUI/diabloui.cpp
+++ b/Source/DiabloUI/diabloui.cpp
@@ -623,6 +623,8 @@ void LoadBackgroundArt(const char *pszFile, int frames)
 	fadeValue = 0;
 	BlackPalette();
 	SDL_FillRect(DiabloUiSurface(), NULL, 0x000000);
+	if (DiabloUiSurface() == pal_surface)
+		BltFast(nullptr, nullptr);
 	RenderPresent();
 }
 
@@ -655,6 +657,8 @@ void UiFadeIn()
 		}
 		SetFadeLevel(fadeValue);
 	}
+	if (DiabloUiSurface() == pal_surface)
+		BltFast(nullptr, nullptr);
 	RenderPresent();
 }
 
diff --git a/Source/DiabloUI/diabloui.h b/Source/DiabloUI/diabloui.h
index 2698f907d..f874def68 100644
--- a/Source/DiabloUI/diabloui.h
+++ b/Source/DiabloUI/diabloui.h
@@ -80,7 +80,18 @@ extern bool (*gfnHeroInfo)(bool (*fninfofunc)(_uiheroinfo *));
 
 inline SDL_Surface *DiabloUiSurface()
 {
-	return GetOutputSurface();
+	auto *output_surface = GetOutputSurface();
+
+#ifdef USE_SDL1
+	// When using a non double-buffered hardware surface, render the UI
+	// to an off-screen surface first to avoid flickering / tearing.
+	if ((output_surface->flags & SDL_HWSURFACE) != 0
+	    && (output_surface->flags & SDL_DOUBLEBUF) == 0) {
+		return pal_surface;
+	}
+#endif
+
+	return output_surface;
 }
 
 void UiDestroy();