From c2c007b356e62044e8b7dcea71bd2ad559570248 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Fri, 17 Sep 2021 14:54:51 +0200
Subject: [PATCH] LibWeb: Don't request WebContent repaint while we have
 repaints pending

This prevents flickering by ensuring that WebContent is only ever
painting into the back buffer bitmap. Without this change, it was
possible for WebContent to paint into the front buffer bitmap.
---
 .../Libraries/LibWeb/OutOfProcessWebView.cpp    | 17 +++++++++--------
 Userland/Libraries/LibWeb/OutOfProcessWebView.h |  1 +
 2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/Userland/Libraries/LibWeb/OutOfProcessWebView.cpp b/Userland/Libraries/LibWeb/OutOfProcessWebView.cpp
index 0ad119df66..4bf06133d5 100644
--- a/Userland/Libraries/LibWeb/OutOfProcessWebView.cpp
+++ b/Userland/Libraries/LibWeb/OutOfProcessWebView.cpp
@@ -128,18 +128,14 @@ void OutOfProcessWebView::handle_resize()
         m_backup_bitmap = m_client_state.front_bitmap.bitmap;
     }
 
-    if (m_client_state.front_bitmap.bitmap) {
-        m_client_state.front_bitmap.bitmap = nullptr;
+    if (m_client_state.front_bitmap.bitmap)
         client().async_remove_backing_store(m_client_state.front_bitmap.id);
-    }
 
-    if (m_client_state.back_bitmap.bitmap) {
-        m_client_state.back_bitmap.bitmap = nullptr;
+    if (m_client_state.back_bitmap.bitmap)
         client().async_remove_backing_store(m_client_state.back_bitmap.id);
-    }
 
-    m_client_state.front_bitmap.id = -1;
-    m_client_state.back_bitmap.id = -1;
+    m_client_state.front_bitmap = {};
+    m_client_state.back_bitmap = {};
     m_client_state.has_usable_bitmap = false;
 
     if (available_size().is_empty())
@@ -201,6 +197,7 @@ void OutOfProcessWebView::notify_server_did_paint(Badge<WebContentClient>, i32 b
 {
     if (m_client_state.back_bitmap.id == bitmap_id) {
         m_client_state.has_usable_bitmap = true;
+        m_client_state.back_bitmap.pending_paints--;
         swap(m_client_state.back_bitmap, m_client_state.front_bitmap);
         // We don't need the backup bitmap anymore, so drop it.
         m_backup_bitmap = nullptr;
@@ -396,6 +393,10 @@ void OutOfProcessWebView::request_repaint()
     // it won't have a back bitmap yet, so we can just skip repaint requests.
     if (!m_client_state.back_bitmap.bitmap)
         return;
+    // Don't request a repaint until pending paint requests have finished.
+    if (m_client_state.back_bitmap.pending_paints)
+        return;
+    m_client_state.back_bitmap.pending_paints++;
     client().async_paint(m_client_state.back_bitmap.bitmap->rect().translated(horizontal_scrollbar().value(), vertical_scrollbar().value()), m_client_state.back_bitmap.id);
 }
 
diff --git a/Userland/Libraries/LibWeb/OutOfProcessWebView.h b/Userland/Libraries/LibWeb/OutOfProcessWebView.h
index 49a74c3f9c..9e70436725 100644
--- a/Userland/Libraries/LibWeb/OutOfProcessWebView.h
+++ b/Userland/Libraries/LibWeb/OutOfProcessWebView.h
@@ -112,6 +112,7 @@ private:
 
     struct SharedBitmap {
         i32 id { -1 };
+        i32 pending_paints { 0 };
         RefPtr<Gfx::Bitmap> bitmap;
     };