From f72a37b9d41b541efa2e2dcaab18e183557d2e68 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Mon, 18 Oct 2021 10:25:13 +0200
Subject: [PATCH] LibWeb: Coalesce nested repaint requests instead of
 swallowing them

Previously we would ignore repaint requests that came in via OOPWV while
the WebContent process was busy with a previous paint request.

This caused some easy-to-trigger bugs where the painted content would be
"one paint behind", especially noticeable when scrolling.
---
 Userland/Libraries/LibWeb/OutOfProcessWebView.cpp | 9 ++++++++-
 Userland/Libraries/LibWeb/OutOfProcessWebView.h   | 1 +
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/Userland/Libraries/LibWeb/OutOfProcessWebView.cpp b/Userland/Libraries/LibWeb/OutOfProcessWebView.cpp
index 49f24646b8..48a5b16243 100644
--- a/Userland/Libraries/LibWeb/OutOfProcessWebView.cpp
+++ b/Userland/Libraries/LibWeb/OutOfProcessWebView.cpp
@@ -207,6 +207,11 @@ void OutOfProcessWebView::notify_server_did_paint(Badge<WebContentClient>, i32 b
         // We don't need the backup bitmap anymore, so drop it.
         m_backup_bitmap = nullptr;
         update();
+
+        if (m_client_state.got_repaint_requests_while_painting) {
+            m_client_state.got_repaint_requests_while_painting = false;
+            request_repaint();
+        }
     }
 }
 
@@ -399,8 +404,10 @@ void OutOfProcessWebView::request_repaint()
     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)
+    if (m_client_state.back_bitmap.pending_paints) {
+        m_client_state.got_repaint_requests_while_painting = true;
         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 7d812a2ecf..818edb2bdf 100644
--- a/Userland/Libraries/LibWeb/OutOfProcessWebView.h
+++ b/Userland/Libraries/LibWeb/OutOfProcessWebView.h
@@ -125,6 +125,7 @@ private:
         SharedBitmap back_bitmap;
         i32 next_bitmap_id { 0 };
         bool has_usable_bitmap { false };
+        bool got_repaint_requests_while_painting { false };
     } m_client_state;
 
     RefPtr<Gfx::Bitmap> m_backup_bitmap;