From 76c39aaf4e0b5ad873c97905f6a936eb2134860c Mon Sep 17 00:00:00 2001
From: Povilas Kanapickas <povilas@radix.lt>
Date: Thu, 21 Jun 2018 00:50:08 +0300
Subject: [PATCH] Accumulate scrolls less than supported scroll on XWindows

This fixes barrier case #67 and synergy case #5670.
---
 src/lib/platform/XWindowsScreen.cpp | 30 +++++++++++++++++------------
 src/lib/platform/XWindowsScreen.h   | 11 +++++++++++
 2 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/src/lib/platform/XWindowsScreen.cpp b/src/lib/platform/XWindowsScreen.cpp
index 581c9111..28464120 100644
--- a/src/lib/platform/XWindowsScreen.cpp
+++ b/src/lib/platform/XWindowsScreen.cpp
@@ -99,6 +99,7 @@ XWindowsScreen::XWindowsScreen(
 		IEventQueue* events) :
 	m_isPrimary(isPrimary),
 	m_mouseScrollDelta(mouseScrollDelta),
+    m_accumulatedScroll(0),
 	m_display(NULL),
 	m_root(None),
 	m_window(None),
@@ -865,9 +866,11 @@ XWindowsScreen::fakeMouseWheel(SInt32, SInt32 yDelta) const
 		return;
 	}
 
-	// choose button depending on rotation direction
-	const unsigned int xButton = mapButtonToX(static_cast<ButtonID>(
-												(yDelta >= 0) ? -1 : -2));
+    int numEvents = accumulateMouseScroll(yDelta);
+
+    // choose button depending on rotation direction
+    const unsigned int xButton = mapButtonToX(static_cast<ButtonID>(
+												(numEvents >= 0) ? -1 : -2));
 	if (xButton == 0) {
 		// If we get here, then the XServer does not support the scroll
 		// wheel buttons, so send PageUp/PageDown keystrokes instead.
@@ -886,20 +889,14 @@ XWindowsScreen::fakeMouseWheel(SInt32, SInt32 yDelta) const
 		return;
 	}
 
-	// now use absolute value of delta
-	if (yDelta < 0) {
-		yDelta = -yDelta;
-	}
-
-	if (yDelta < m_mouseScrollDelta) {
-		LOG((CLOG_WARN "Wheel scroll delta (%d) smaller than threshold (%d)", yDelta, m_mouseScrollDelta));
-	}
+    numEvents = std::abs(numEvents);
 
 	// send as many clicks as necessary
-	for (; yDelta >= m_mouseScrollDelta; yDelta -= m_mouseScrollDelta) {
+    for (; numEvents > 0; numEvents--) {
 		XTestFakeButtonEvent(m_display, xButton, True, CurrentTime);
 		XTestFakeButtonEvent(m_display, xButton, False, CurrentTime);
 	}
+
 	XFlush(m_display);
 }
 
@@ -1643,6 +1640,15 @@ XWindowsScreen::onMouseMove(const XMotionEvent& xmotion)
 	}
 }
 
+int
+XWindowsScreen::accumulateMouseScroll(SInt32 yDelta) const
+{
+    m_accumulatedScroll += yDelta;
+    int numEvents = m_accumulatedScroll / m_mouseScrollDelta;
+    m_accumulatedScroll -= numEvents * m_mouseScrollDelta;
+    return numEvents;
+}
+
 Cursor
 XWindowsScreen::createBlankCursor() const
 {
diff --git a/src/lib/platform/XWindowsScreen.h b/src/lib/platform/XWindowsScreen.h
index 35f9368e..a2e3495b 100644
--- a/src/lib/platform/XWindowsScreen.h
+++ b/src/lib/platform/XWindowsScreen.h
@@ -136,6 +136,10 @@ private:
     void                onMouseRelease(const XButtonEvent&);
     void                onMouseMove(const XMotionEvent&);
 
+    // Returns the number of scroll events needed after the current delta has
+    // been taken into account
+    int                 accumulateMouseScroll(SInt32 yDelta) const;
+
     bool                detectXI2();
 #ifdef HAVE_XI2
     void                selectXIRawMotion();
@@ -172,8 +176,15 @@ private:
 
     // true if screen is being used as a primary screen, false otherwise
     bool                m_isPrimary;
+
+    // The size of a smallest supported scroll event, in points
     int                 m_mouseScrollDelta;
 
+    // Accumulates scrolls of less than m_mouseScrollDelta across multiple
+    // scroll events. We dispatch a scroll event whenever the accumulated scroll
+    // becomes larger than m_mouseScrollDelta
+    mutable int         m_accumulatedScroll;
+
     Display*            m_display;
     Window                m_root;
     Window                m_window;