Account for WHEEL_DELTA when dispatching VT mouse wheel events

src/interactivity/win32/windowio.cpp

@@ -630,20 +630,7 @@ BOOL HandleMouseEvent(const SCREEN_INFORMATION& ScreenInfo,
         short sDelta = 0;
         if (Message == WM_MOUSEWHEEL)
         {
-            short sWheelDelta = GET_WHEEL_DELTA_WPARAM(wParam);
-            // For most devices, we'll get mouse events as multiples of
-            // WHEEL_DELTA, where WHEEL_DELTA represents a single scroll unit
-            // But sometimes, things like trackpads will scroll in finer
-            // measurements. In this case, the VT mouse scrolling wouldn't work.
-            // So if that happens, ensure we scroll at least one time.
-            if (abs(sWheelDelta) < WHEEL_DELTA)
-            {
-                sDelta = sWheelDelta < 0 ? -1 : 1;
-            }
-            else
-            {
-                sDelta = sWheelDelta / WHEEL_DELTA;
-            }
+            sDelta = GET_WHEEL_DELTA_WPARAM(wParam);
         }
 
         if (HandleTerminalMouseEvent(MousePosition, Message, GET_KEYSTATE_WPARAM(wParam), sDelta))

src/terminal/adapter/ut_adapter/MouseInputTest.cpp

@@ -514,7 +514,7 @@ class MouseInputTest
     TEST_METHOD(ScrollWheelTests)
     {
         BEGIN_TEST_METHOD_PROPERTIES()
-            TEST_METHOD_PROPERTY(L"Data:sScrollDelta", L"{0, -1, 1, 100, -10000, 32736}")
+            TEST_METHOD_PROPERTY(L"Data:sScrollDelta", L"{-120, 120, -10000, 32736}")
             TEST_METHOD_PROPERTY(L"Data:uiModifierKeystate", L"{0x0000, 0x0004, 0x0008}")
         END_TEST_METHOD_PROPERTIES()
 
@@ -606,31 +606,31 @@ class MouseInputTest
 
         Log::Comment(L"Test mouse wheel scrolling up");
         s_pwszInputExpected = L"\x1B[A";
-        VERIFY_IS_TRUE(mouseInput->HandleMouse({ 0, 0 }, WM_MOUSEWHEEL, noModifierKeys, 1));
+        VERIFY_IS_TRUE(mouseInput->HandleMouse({ 0, 0 }, WM_MOUSEWHEEL, noModifierKeys, WHEEL_DELTA));
 
         Log::Comment(L"Test mouse wheel scrolling down");
         s_pwszInputExpected = L"\x1B[B";
-        VERIFY_IS_TRUE(mouseInput->HandleMouse({ 0, 0 }, WM_MOUSEWHEEL, noModifierKeys, -1));
+        VERIFY_IS_TRUE(mouseInput->HandleMouse({ 0, 0 }, WM_MOUSEWHEEL, noModifierKeys, -WHEEL_DELTA));
 
         Log::Comment(L"Enable cursor keys mode");
         mouseInput->ChangeCursorKeysMode(true);
 
         Log::Comment(L"Test mouse wheel scrolling up");
         s_pwszInputExpected = L"\x1BOA";
-        VERIFY_IS_TRUE(mouseInput->HandleMouse({ 0, 0 }, WM_MOUSEWHEEL, noModifierKeys, 1));
+        VERIFY_IS_TRUE(mouseInput->HandleMouse({ 0, 0 }, WM_MOUSEWHEEL, noModifierKeys, WHEEL_DELTA));
 
         Log::Comment(L"Test mouse wheel scrolling down");
         s_pwszInputExpected = L"\x1BOB";
-        VERIFY_IS_TRUE(mouseInput->HandleMouse({ 0, 0 }, WM_MOUSEWHEEL, noModifierKeys, -1));
+        VERIFY_IS_TRUE(mouseInput->HandleMouse({ 0, 0 }, WM_MOUSEWHEEL, noModifierKeys, -WHEEL_DELTA));
 
         Log::Comment(L"Confirm no effect when scroll mode is disabled");
         mouseInput->UseAlternateScreenBuffer();
         mouseInput->EnableAlternateScroll(false);
-        VERIFY_IS_FALSE(mouseInput->HandleMouse({ 0, 0 }, WM_MOUSEWHEEL, noModifierKeys, 1));
+        VERIFY_IS_FALSE(mouseInput->HandleMouse({ 0, 0 }, WM_MOUSEWHEEL, noModifierKeys, WHEEL_DELTA));
 
         Log::Comment(L"Confirm no effect when using the main buffer");
         mouseInput->UseMainScreenBuffer();
         mouseInput->EnableAlternateScroll(true);
-        VERIFY_IS_FALSE(mouseInput->HandleMouse({ 0, 0 }, WM_MOUSEWHEEL, noModifierKeys, 1));
+        VERIFY_IS_FALSE(mouseInput->HandleMouse({ 0, 0 }, WM_MOUSEWHEEL, noModifierKeys, WHEEL_DELTA));
     }
 };

src/terminal/input/mouseInput.cpp

@@ -4,6 +4,7 @@
 #include "precomp.h"
 #include <windows.h>
 #include "terminalInput.hpp"
+#include "../types/inc/utils.hpp"
 
 using namespace Microsoft::Console::VirtualTerminal;
 
@@ -63,6 +64,17 @@ static constexpr bool _isHoverMsg(const unsigned int buttonCode) noexcept
     return buttonCode == WM_MOUSEMOVE;
 }
 
+// Routine Description:
+// - Determines if the input windows message code describes a mouse wheel event
+// Parameters:
+// - buttonCode - the message to decode.
+// Return value:
+// - true iff buttonCode is a wheel event to translate
+static constexpr bool _isWheelMsg(const unsigned int buttonCode) noexcept
+{
+    return buttonCode == WM_MOUSEWHEEL || buttonCode == WM_MOUSEHWHEEL;
+}
+
 // Routine Description:
 // - Determines if the input windows message code describes a button press
 //     (either down or doubleclick)
@@ -293,6 +305,32 @@ bool TerminalInput::HandleMouse(const COORD position,
                                 const short modifierKeyState,
                                 const short delta)
 {
+    if (Utils::Sign(delta) != Utils::Sign(_mouseInputState.accumulatedDelta))
+    {
+        // This works for wheel and non-wheel events and transitioning between wheel/non-wheel.
+        // Non-wheel events have a delta of 0, which will fail to match the sign on
+        // a real wheel event or the accumulated delta. Wheel events will be either + or -
+        // and we only want to accumulate them if they match in sign.
+        _mouseInputState.accumulatedDelta = 0;
+    }
+
+    if (_isWheelMsg(button))
+    {
+        _mouseInputState.accumulatedDelta += delta;
+        if (std::abs(_mouseInputState.accumulatedDelta) < WHEEL_DELTA)
+        {
+            // If we're accumulating button presses of the same type, *and* those presses are
+            // on the wheel, accumulate delta until we hit the amount required to dispatch one
+            // "line" worth of scroll.
+            // Mark the event as "handled" if we would have otherwise emitted a scroll event.
+            return IsTrackingMouseInput() || _ShouldSendAlternateScroll(button, delta);
+        }
+
+        // We're ready to send this event through, but first we need to clear the accumulated;
+        // delta. Otherwise, we'll dispatch every subsequent sub-delta event as its own event.
+        _mouseInputState.accumulatedDelta = 0;
+    }
+
     bool success = false;
     if (_ShouldSendAlternateScroll(button, delta))
     {

src/terminal/input/terminalInput.hpp

@@ -109,6 +109,7 @@ namespace Microsoft::Console::VirtualTerminal
             bool inAlternateBuffer{ false };
             COORD lastPos{ -1, -1 };
             unsigned int lastButton{ 0 };
+            int accumulatedDelta{ 0 };
         };
 
         MouseInputState _mouseInputState;

src/types/inc/utils.hpp

@@ -15,6 +15,14 @@ Author(s):
 
 namespace Microsoft::Console::Utils
 {
+    // Function Description:
+    // - Returns -1, 0 or +1 to indicate the sign of the passed-in value.
+    template<typename T>
+    constexpr int Sign(T val) noexcept
+    {
+        return (T{ 0 } < val) - (val < T{ 0 });
+    }
+
     bool IsValidHandle(const HANDLE handle) noexcept;
 
     // Function Description: