Move scroll timer to a thread, instead of SetTimer

src/Canvas.cpp

@@ -61,9 +61,6 @@ bool gNoFlickerRender = true;
 
 Kind kNotifAnnotation = "notifAnnotation";
 
-// Timer for mouse wheel smooth scrolling
-constexpr UINT_PTR kSmoothScrollTimerID = 6;
-
 // Smooth scrolling factor. This is a value between 0 and 1.
 // Each step, we scroll the needed delta times this factor.
 // Therefore, a higher factor makes smooth scrolling faster.
@@ -151,7 +148,7 @@ static void OnVScroll(MainWindow* win, WPARAM wp) {
     if (si.nPos != currPos || msg == SB_THUMBTRACK) {
         if (gGlobalPrefs->smoothScroll) {
             win->scrollTargetY = si.nPos;
-            SetTimer(win->hwndCanvas, kSmoothScrollTimerID, USER_TIMER_MINIMUM, nullptr);
+            SetEvent(win->scrollTimer);
         } else {
             win->AsFixed()->ScrollYTo(si.nPos);
         }
@@ -1211,7 +1208,7 @@ static void ZoomByMouseWheel(MainWindow* win, WPARAM wp) {
     win->dragStartPending = false;
     // Kill the smooth scroll timer when zooming
     // We don't want to move to the new updated y offset after zooming
-    KillTimer(win->hwndCanvas, kSmoothScrollTimerID);
+    ResetEvent(win->scrollTimer);
 
     short delta = GET_WHEEL_DELTA_WPARAM(wp);
     Point pt = HwndGetCursorPos(win->hwndCanvas);
@@ -1875,7 +1872,7 @@ static void OnTimer(MainWindow* win, HWND hwnd, WPARAM timerId) {
             int delta = target - current;
 
             if (delta == 0) {
-                KillTimer(hwnd, kSmoothScrollTimerID);
+                ResetEvent(win->scrollTimer);
             } else {
                 // logf("Smooth scrolling from %d to %d (delta %d)\n", current, target, delta);
 

src/Canvas.h

@@ -1,6 +1,9 @@
 /* Copyright 2022 the SumatraPDF project authors (see AUTHORS file).
    License: GPLv3 */
 
+// Timer for mouse wheel smooth scrolling
+constexpr UINT_PTR kSmoothScrollTimerID = 6;
+
 void UpdateDeltaPerLine();
 
 LRESULT CALLBACK WndProcCanvas(HWND, UINT, WPARAM, LPARAM);

src/MainWindow.h

@@ -212,6 +212,11 @@ struct MainWindow {
 
     DocControllerCallback* cbHandler = nullptr;
 
+    // Seprate scroll thread for higher precision scrolling updates
+    HANDLE scrollTimerThread = nullptr;
+    bool scrollTimerCancelled = false;
+    HANDLE scrollTimer = nullptr;
+
     // The target y offset for smooth scrolling.
     // We use a timer to gradually scroll there.
     int scrollTargetY = 0;

src/SumatraPDF.cpp

@@ -1464,6 +1464,36 @@ static void UpdateToolbarSidebarText(MainWindow* win) {
     win->favLabelWithClose->SetLabel(_TRA("Favorites"));
 }
 
+static void ScrollTimerThread(MainWindow* win) {
+    while (WaitForSingleObject(win->scrollTimer, INFINITE) == WAIT_OBJECT_0) {
+        if (win->scrollTimerCancelled) {
+            break;
+        }
+
+        SendMessage(win->hwndCanvas, WM_TIMER, kSmoothScrollTimerID, 0);
+
+        // Re-fetch the monitor refresh rate in case window is moved to
+        // new window, or monitor refresh rate changed
+
+        HMONITOR monitor = MonitorFromWindow(win->hwndCanvas, MONITOR_DEFAULTTOPRIMARY);
+        MONITORINFOEX info;
+        info.cbSize = sizeof(MONITORINFOEX);
+        if (!GetMonitorInfo(monitor, &info)) {
+            Sleep(1000 / 60);
+            continue;
+        }
+        DEVMODE mode;
+        mode.dmSize = sizeof(DEVMODE);
+        mode.dmDriverExtra = 0;
+        if (!EnumDisplaySettings(info.szDevice, ENUM_CURRENT_SETTINGS, &mode) || mode.dmDisplayFrequency == 0) {
+            Sleep(1000 / 60);
+            continue;
+        }
+
+        Sleep(1000 / mode.dmDisplayFrequency);
+    }
+}
+
 static MainWindow* CreateMainWindow() {
     Rect windowPos = gGlobalPrefs->windowPos;
     if (!windowPos.IsEmpty()) {
@@ -1513,6 +1543,10 @@ static MainWindow* CreateMainWindow() {
     // hide scrollbars to avoid showing/hiding on empty window
     ShowScrollBar(win->hwndCanvas, SB_BOTH, FALSE);
 
+    win->scrollTimer = CreateEvent(nullptr, true, false, nullptr);
+    Func0 fn = MkFunc0(ScrollTimerThread, win);
+    win->scrollTimerThread = StartThread(fn, "Scroll Thread");
+
     ReportIf(win->menu);
     win->menu = BuildMenu(win);
     win->isMenuHidden = !gGlobalPrefs->showMenubar;
@@ -2728,6 +2762,12 @@ void CloseWindow(MainWindow* win, bool quitIfLast, bool forceClose) {
 
     AbortFinding(win, true);
     AbortPrinting(win);
+    if (win->scrollTimerThread) {
+        win->scrollTimerCancelled = true;
+        SetEvent(win->scrollTimer);
+        WaitForSingleObject(win->scrollTimerThread, INFINITE);
+    }
+    win->scrollTimerCancelled = false;
 
     for (auto& tab : win->Tabs()) {
         if (tab->AsFixed()) {