From 722b7d6cc4117eac900dd0fb51f451f830ce7605 Mon Sep 17 00:00:00 2001
From: Povilas Kanapickas <povilas@radix.lt>
Date: Wed, 20 May 2020 22:11:14 +0300
Subject: [PATCH] XWindowsEventQueueBuffer: Fix delays when waiting for new
 events

QLength() may return 0 even if there are events pending because they
need to be read from the display socket in order to become visible. We
must use XPending() which will poll the socket if QLength() == 0.
---
 src/lib/platform/XWindowsEventQueueBuffer.cpp | 15 ++++++++++++++-
 src/lib/platform/XWindowsEventQueueBuffer.h   |  2 ++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/lib/platform/XWindowsEventQueueBuffer.cpp b/src/lib/platform/XWindowsEventQueueBuffer.cpp
index 78f0e5af..8b8c3b59 100644
--- a/src/lib/platform/XWindowsEventQueueBuffer.cpp
+++ b/src/lib/platform/XWindowsEventQueueBuffer.cpp
@@ -82,6 +82,19 @@ XWindowsEventQueueBuffer::~XWindowsEventQueueBuffer()
     close(m_pipefd[1]);
 }
 
+int XWindowsEventQueueBuffer::getPendingCountLocked()
+{
+    Lock lock(&m_mutex);
+    // work around a bug in old libx11 which causes the first XPending not to read events under
+    // certain conditions. The issue happens when libx11 has not yet received replies for all
+    // flushed events. In that case, internally XPending will not try to process received events
+    // as the reply for the last event was not found. As a result, XPending will return the number
+    // of pending events without regard to the events it has just read.
+    // https://gitlab.freedesktop.org/xorg/lib/libx11/-/merge_requests/1 fixes this on libx11 side.
+    m_impl->XPending(m_display);
+    return m_impl->XPending(m_display);
+}
+
 void
 XWindowsEventQueueBuffer::waitForEvent(double dtimeout)
 {
@@ -163,7 +176,7 @@ XWindowsEventQueueBuffer::waitForEvent(double dtimeout)
     // we want to give the cpu a chance s owe up this to 25ms
 #define TIMEOUT_DELAY 25
 
-    while (((dtimeout < 0.0) || (remaining > 0)) && QLength(m_display)==0 && retval==0){
+    while (((dtimeout < 0.0) || (remaining > 0)) && getPendingCountLocked() == 0 && retval == 0) {
 #if HAVE_POLL
     retval = poll(pfds, 2, TIMEOUT_DELAY); //16ms = 60hz, but we make it > to play nicely with the cpu
      if (pfds[1].revents & POLLIN) {
diff --git a/src/lib/platform/XWindowsEventQueueBuffer.h b/src/lib/platform/XWindowsEventQueueBuffer.h
index e49b282b..13f6b160 100644
--- a/src/lib/platform/XWindowsEventQueueBuffer.h
+++ b/src/lib/platform/XWindowsEventQueueBuffer.h
@@ -51,6 +51,8 @@ public:
 private:
     void                flush();
 
+    int getPendingCountLocked();
+
 private:
     typedef std::vector<XEvent> EventList;
      IXWindowsImpl*       m_impl;