From 5c1fc77f9bdca03065802753ec29efa65bc6d01a Mon Sep 17 00:00:00 2001
From: Stephen Heumann <stephenheumann@gmail.com>
Date: Wed, 21 Sep 2016 18:30:03 -0500
Subject: [PATCH] Don't request another FBUpdate in NextRect if one is already
 coming.

This helps avoid issues where the GS may be unable to "catch up" with the stream of FBUpdates, resulting in it continually drawing out-of-date screen data.
---
 vncdisplay.cc | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/vncdisplay.cc b/vncdisplay.cc
index 137f43a..2992486 100644
--- a/vncdisplay.cc
+++ b/vncdisplay.cc
@@ -39,6 +39,7 @@ unsigned int fbHeight;
 unsigned int fbWidth;
 
 BOOLEAN displayInProgress;
+static BOOLEAN peekedNextMsg;
 
 static unsigned int numRects;
 unsigned int rectX;
@@ -66,6 +67,12 @@ BOOLEAN checkBounds = FALSE;    /* Adjust drawing to stay in bounds */
 
 unsigned long skipBytes = 0;
 
+/* Server-to-client message types */
+#define FBUpdate            0
+#define SetColourMapEntries 1
+#define Bell                2
+#define ServerCutText       3
+
 #define txtColor        10
 #define txtGray         11
 #define txtTransfers    23
@@ -187,6 +194,7 @@ void InitVNCWindow(void) {
     /* We also take the opportunity here to initialize the rectangle info. */
     numRects = 0;
     displayInProgress = FALSE;
+    peekedNextMsg = FALSE;
 
 #undef wrNum320
 #undef wrNum640
@@ -288,6 +296,16 @@ void NextRect (void) {
         Origin contentOrigin;
 
         DoneWithReadBuffer();
+        
+        if (DoReadTCP(1)) {
+            peekedNextMsg = TRUE;
+            if (*readBufferPtr == FBUpdate) {
+                /* Don't request another FBUpdate if one is already coming.
+                 * This helps avoid a condition where the GS may be unable
+                 * to "catch up" with a stream of frequent updates. */
+                return;
+            }
+        }
 
         contentOrigin.l = GetContentOrigin(vncWindow);
         SendFBUpdateRequest(TRUE, contentOrigin.pt.h, contentOrigin.pt.v,
@@ -297,10 +315,6 @@ void NextRect (void) {
     
 void ConnectedEventLoop (void) {
     unsigned char messageType;
-#define FBUpdate            0
-#define SetColourMapEntries 1
-#define Bell                2
-#define ServerCutText       3
 
     if (FrontWindow() != vncWindow && menuOffset == noKB)
         InitMenus(0);
@@ -346,7 +360,8 @@ void ConnectedEventLoop (void) {
                                 return;
         }
     }
-    else if (DoReadTCP(1)) {            /* Read message type byte */
+    else if (peekedNextMsg || DoReadTCP(1)) {   /* Read message type byte */
+        peekedNextMsg = FALSE;
         messageType = *readBufferPtr;
         switch (messageType) {
             case FBUpdate:              DoFBUpdate();