[Scroll Wheel] Emulate discrete scrolling from v120 events

src/config/ConfigManager.cpp

@@ -472,6 +472,7 @@ CConfigManager::CConfigManager() {
     m_pConfig->addConfigValue("input:scroll_button_lock", Hyprlang::INT{0});
     m_pConfig->addConfigValue("input:scroll_factor", {1.f});
     m_pConfig->addConfigValue("input:scroll_points", {STRVAL_EMPTY});
+    m_pConfig->addConfigValue("input:emulate_discrete_scroll", Hyprlang::INT{1});
     m_pConfig->addConfigValue("input:touchpad:natural_scroll", Hyprlang::INT{0});
     m_pConfig->addConfigValue("input:touchpad:disable_while_typing", Hyprlang::INT{1});
     m_pConfig->addConfigValue("input:touchpad:clickfinger_behavior", Hyprlang::INT{0});

src/managers/SeatManager.cpp

@@ -333,9 +333,7 @@ void CSeatManager::sendPointerAxis(uint32_t timeMs, wl_pointer_axis axis, double
             if (source == 0) {
                 p->sendAxisValue120(axis, value120);
                 p->sendAxisDiscrete(axis, discrete);
-            }
-
-            if (value == 0)
+            } else if (value == 0)
                 p->sendAxisStop(timeMs, axis);
         }
     }

src/managers/input/InputManager.cpp

@@ -760,6 +760,7 @@ void CInputManager::onMouseWheel(IPointer::SAxisEvent e) {
     static auto POFFWINDOWAXIS        = CConfigValue<Hyprlang::INT>("input:off_window_axis_events");
     static auto PINPUTSCROLLFACTOR    = CConfigValue<Hyprlang::FLOAT>("input:scroll_factor");
     static auto PTOUCHPADSCROLLFACTOR = CConfigValue<Hyprlang::FLOAT>("input:touchpad:scroll_factor");
+    static auto PEMULATEDISCRETE      = CConfigValue<Hyprlang::INT>("input:emulate_discrete_scroll");
 
     auto        factor = (*PTOUCHPADSCROLLFACTOR <= 0.f || e.source == WL_POINTER_AXIS_SOURCE_FINGER ? *PTOUCHPADSCROLLFACTOR : *PINPUTSCROLLFACTOR);
 
@@ -798,9 +799,44 @@ void CInputManager::onMouseWheel(IPointer::SAxisEvent e) {
             }
         }
     }
-    double deltaDiscrete = (e.deltaDiscrete != 0) ? (factor * e.deltaDiscrete / std::abs(e.deltaDiscrete)) : 0;
-    g_pSeatManager->sendPointerAxis(e.timeMs, e.axis, factor * e.delta, deltaDiscrete > 0 ? std::ceil(deltaDiscrete) : std::floor(deltaDiscrete),
-                                    std::round(factor * e.deltaDiscrete), e.source, WL_POINTER_AXIS_RELATIVE_DIRECTION_IDENTICAL);
+
+    double discrete = (e.deltaDiscrete != 0) ? (factor * e.deltaDiscrete / std::abs(e.deltaDiscrete)) : 0;
+    double delta    = e.delta * factor;
+
+    if (e.source == 0) {
+        // if an application supports v120, it should ignore discrete anyways
+        if ((*PEMULATEDISCRETE >= 1 && std::abs(e.deltaDiscrete) != 120) || *PEMULATEDISCRETE >= 2) {
+
+            const int interval = factor != 0 ? std::round(120 * (1 / factor)) : 120;
+
+            // reset the accumulator when timeout is reached or direction/axis has changed
+            if (std::signbit(e.deltaDiscrete) != m_ScrollWheelState.lastEventSign || e.axis != m_ScrollWheelState.lastEventAxis ||
+                e.timeMs - m_ScrollWheelState.lastEventTime > 500 /* 500ms taken from libinput default timeout */) {
+
+                m_ScrollWheelState.accumulatedScroll = 0;
+                // send 1 discrete on first event for responsiveness
+                discrete = std::copysign(1, e.deltaDiscrete);
+            } else
+                discrete = 0;
+
+            for (int ac = m_ScrollWheelState.accumulatedScroll; ac >= interval; ac -= interval) {
+                discrete += std::copysign(1, e.deltaDiscrete);
+                m_ScrollWheelState.accumulatedScroll -= interval;
+            }
+
+            m_ScrollWheelState.lastEventSign = std::signbit(e.deltaDiscrete);
+            m_ScrollWheelState.lastEventAxis = e.axis;
+            m_ScrollWheelState.lastEventTime = e.timeMs;
+            m_ScrollWheelState.accumulatedScroll += std::abs(e.deltaDiscrete);
+
+            delta = 15.0 * discrete * factor;
+        }
+    }
+
+    int32_t value120      = std::round(factor * e.deltaDiscrete);
+    int32_t deltaDiscrete = std::abs(discrete) != 0 && std::abs(discrete) < 1 ? std::copysign(1, discrete) : std::round(discrete);
+
+    g_pSeatManager->sendPointerAxis(e.timeMs, e.axis, delta, deltaDiscrete, value120, e.source, WL_POINTER_AXIS_RELATIVE_DIRECTION_IDENTICAL);
 }
 
 Vector2D CInputManager::getMouseCoordsInternal() {

src/managers/input/InputManager.hpp

@@ -278,6 +278,14 @@ class CInputManager {
 
     void restoreCursorIconToApp(); // no-op if restored
 
+    // discrete scrolling emulation using v120 data
+    struct {
+        bool     lastEventSign     = 0;
+        bool     lastEventAxis     = 0;
+        uint32_t lastEventTime     = 0;
+        uint32_t accumulatedScroll = 0;
+    } m_ScrollWheelState;
+
     friend class CKeybindManager;
     friend class CWLSurface;
 };