LibGUI+WindowServer: Coalesce paints and resizes on the client side.

Only process paint and resize events on the GUI client side if those events
have the latest up-to-date window size. This drastically reduces async
overdraw during interactive resize.

2 files changed, 48 insertions, 1 deletions

diff --git a/LibGUI/GEventLoop.cpp b/LibGUI/GEventLoop.cpp
index b91f4f9f86..e60ac985d8 100644
--- a/LibGUI/GEventLoop.cpp
+++ b/LibGUI/GEventLoop.cpp
@@ -20,6 +20,7 @@
 #include <LibC/stdlib.h>
 
 //#define GEVENTLOOP_DEBUG
+//#define COALESCING_DEBUG
 
 static HashMap<GShortcut, GAction*>* g_actions;
 static GEventLoop* s_main_event_loop;
@@ -352,7 +353,32 @@ void GEventLoop::wait_for_event()
 
 void GEventLoop::process_unprocessed_messages()
 {
+    int coalesced_paints = 0;
+    int coalesced_resizes = 0;
     auto unprocessed_events = move(m_unprocessed_messages);
+
+    HashMap<int, Size> latest_size_for_window_id;
+    for (auto& event : unprocessed_events) {
+        if (event.type == WSAPI_ServerMessage::Type::WindowResized) {
+            latest_size_for_window_id.set(event.window_id, event.window.rect.size);
+        }
+    }
+
+    int paint_count = 0;
+    HashMap<int, Size> latest_paint_size_for_window_id;
+    for (auto& event : unprocessed_events) {
+        if (event.type == WSAPI_ServerMessage::Type::Paint) {
+            ++paint_count;
+#ifdef COALESCING_DEBUG
+            dbgprintf("    %s (window: %s)\n", Rect(event.paint.rect).to_string().characters(), Size(event.paint.window_size).to_string().characters());
+#endif
+            latest_paint_size_for_window_id.set(event.window_id, event.paint.window_size);
+        }
+    }
+#ifdef COALESCING_DEBUG
+    dbgprintf("paint_count: %d\n", paint_count);
+#endif
+
     for (auto& event : unprocessed_events) {
         if (event.type == WSAPI_ServerMessage::Type::Greeting) {
             s_server_pid = event.greeting.server_pid;
@@ -387,6 +413,10 @@ void GEventLoop::process_unprocessed_messages()
         }
         switch (event.type) {
         case WSAPI_ServerMessage::Type::Paint:
+            if (Size(event.paint.window_size) != latest_paint_size_for_window_id.get(event.window_id)) {
+                ++coalesced_paints;
+                break;
+            }
             handle_paint_event(event, *window);
             break;
         case WSAPI_ServerMessage::Type::MouseDown:
@@ -410,6 +440,10 @@ void GEventLoop::process_unprocessed_messages()
             handle_window_entered_or_left_event(event, *window);
             break;
         case WSAPI_ServerMessage::Type::WindowResized:
+            if (Size(event.window.rect.size) != latest_size_for_window_id.get(event.window_id)) {
+                ++coalesced_resizes;
+                break;
+            }
             handle_resize_event(event, *window);
             break;
         case WSAPI_ServerMessage::Type::WM_WindowRemoved:
@@ -421,6 +455,13 @@ void GEventLoop::process_unprocessed_messages()
         }
     }
 
+#ifdef COALESCING_DEBUG
+    if (coalesced_paints)
+        dbgprintf("Coalesced %d paints\n", coalesced_paints);
+    if (coalesced_resizes)
+        dbgprintf("Coalesced %d resizes\n", coalesced_resizes);
+#endif
+
     if (!m_unprocessed_messages.is_empty())
         process_unprocessed_messages();
 }
diff --git a/LibGUI/GWindow.cpp b/LibGUI/GWindow.cpp
index d2f41176ef..1c47ec3c6c 100644
--- a/LibGUI/GWindow.cpp
+++ b/LibGUI/GWindow.cpp
@@ -211,11 +211,17 @@ void GWindow::event(GEvent& event)
             return;
         auto& paint_event = static_cast<GPaintEvent&>(event);
         auto rect = paint_event.rect();
+        if (m_back_bitmap && m_back_bitmap->size() != paint_event.window_size()) {
+            // Eagerly discard the backing store if we learn from this paint event that it needs to be bigger.
+            // Otherwise we would have to wait for a resize event to tell us. This way we don't waste the
+            // effort on painting into an undersized bitmap that will be thrown away anyway.
+            m_back_bitmap = nullptr;
+        }
         bool created_new_backing_store = !m_back_bitmap;
         if (!m_back_bitmap)
             m_back_bitmap = create_backing_bitmap(paint_event.window_size());
         if (rect.is_empty() || created_new_backing_store)
-            rect = m_main_widget->rect();
+            rect = { { }, paint_event.window_size() };
 
         m_main_widget->event(*make<GPaintEvent>(rect));