Add support for alternate scroll mode in Terminal

src/cascadia/TerminalControl/ControlCore.cpp

@@ -1381,6 +1381,11 @@ namespace winrt::Microsoft::Terminal::Control::implementation
     {
         return _terminal != nullptr && _terminal->IsTrackingMouseInput();
     }
+    bool ControlCore::ShouldSendAlternateScroll(const unsigned int uiButton,
+                                                const int32_t delta) const
+    {
+        return _terminal != nullptr && _terminal->ShouldSendAlternateScroll(uiButton, delta);
+    }
 
     Core::Point ControlCore::CursorPosition() const
     {

src/cascadia/TerminalControl/ControlCore.h

@@ -138,6 +138,7 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         void CursorOn(const bool isCursorOn);
 
         bool IsVtMouseModeEnabled() const;
+        bool ShouldSendAlternateScroll(const unsigned int uiButton, const int32_t delta) const;
         Core::Point CursorPosition() const;
 
         bool HasSelection() const;

src/cascadia/TerminalControl/ControlInteractivity.cpp

@@ -409,8 +409,14 @@ namespace winrt::Microsoft::Terminal::Control::implementation
     {
         const til::point terminalPosition = _getTerminalPosition(til::point{ pixelPosition });
 
-        // Short-circuit isReadOnly check to avoid warning dialog
-        if (!_core->IsInReadOnlyMode() && _canSendVTMouseInput(modifiers))
+        // Short-circuit isReadOnly check to avoid warning dialog.
+        //
+        // GH#3321: Alternate scroll mode is a special type of mouse input mode
+        // where the terminal sends arrow keys when the user mouse wheels, but
+        // the client app doesn't care for other mouse input. It's tracked
+        // separately from _canSendVTMouseInput.
+        if (!_core->IsInReadOnlyMode() &&
+            (_canSendVTMouseInput(modifiers) || _shouldSendAlternateScroll(modifiers, delta)))
         {
             // Most mouse event handlers call
             //      _trySendMouseEvent(point);
@@ -571,6 +577,17 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         return _core->IsVtMouseModeEnabled();
     }
 
+    bool ControlInteractivity::_shouldSendAlternateScroll(const ::Microsoft::Terminal::Core::ControlKeyStates modifiers, const int32_t delta)
+    {
+        // If the user is holding down Shift, suppress mouse events
+        // TODO GH#4875: disable/customize this functionality
+        if (modifiers.IsShiftPressed())
+        {
+            return false;
+        }
+        return _core->ShouldSendAlternateScroll(WM_MOUSEWHEEL, delta);
+    }
+
     // Method Description:
     // - Sets selection's end position to match supplied cursor position, e.g. while mouse dragging.
     // Arguments:

src/cascadia/TerminalControl/ControlInteractivity.h

@@ -140,6 +140,7 @@ namespace winrt::Microsoft::Terminal::Control::implementation
 
         void _hyperlinkHandler(const std::wstring_view uri);
         bool _canSendVTMouseInput(const ::Microsoft::Terminal::Core::ControlKeyStates modifiers);
+        bool _shouldSendAlternateScroll(const ::Microsoft::Terminal::Core::ControlKeyStates modifiers, const int32_t delta);
 
         void _sendPastedTextToConnection(std::wstring_view wstr);
         til::point _getTerminalPosition(const til::point pixelPosition);

src/cascadia/TerminalCore/Terminal.cpp

@@ -536,6 +536,20 @@ bool Terminal::IsTrackingMouseInput() const noexcept
     return _terminalInput->IsTrackingMouseInput();
 }
 
+// Routine Description:
+// - Relays if we are in alternate scroll mode, a special type of mouse input
+//   mode where scrolling sends the arrow keypresses, but the app doesn't
+//   otherwise want mouse input.
+// Parameters:
+// - <none>
+// Return value:
+// - true, if we are tracking mouse input. False, otherwise
+bool Terminal::ShouldSendAlternateScroll(const unsigned int uiButton,
+                                         const int32_t delta) const noexcept
+{
+    return _terminalInput->ShouldSendAlternateScroll(uiButton, ::base::saturated_cast<short>(delta));
+}
+
 // Method Description:
 // - Given a coord, get the URI at that location
 // Arguments:

src/cascadia/TerminalCore/Terminal.hpp

@@ -154,6 +154,7 @@ class Microsoft::Terminal::Core::Terminal final :
 
     void TrySnapOnInput() override;
     bool IsTrackingMouseInput() const noexcept;
+    bool ShouldSendAlternateScroll(const unsigned int uiButton, const int32_t delta) const noexcept;
 
     std::wstring GetHyperlinkAtPosition(const COORD position);
     uint16_t GetHyperlinkIdAtPosition(const COORD position);

src/cascadia/TerminalCore/TerminalApi.cpp

@@ -630,6 +630,10 @@ void Terminal::UseAlternateScreenBuffer()
     // update all the hyperlinks on the screen
     _updateUrlDetection();
 
+    // GH#3321: Make sure we let the TerminalInput know that we switched
+    // buffers. This might affect how we interpret certain mouse events.
+    _terminalInput->UseAlternateScreenBuffer();
+
     // Update scrollbars
     _NotifyScrollEvent();
 
@@ -674,6 +678,10 @@ void Terminal::UseMainScreenBuffer()
     _mainBuffer->ClearPatternRecognizers();
     _updateUrlDetection();
 
+    // GH#3321: Make sure we let the TerminalInput know that we switched
+    // buffers. This might affect how we interpret certain mouse events.
+    _terminalInput->UseMainScreenBuffer();
+
     // Update scrollbars
     _NotifyScrollEvent();
 

src/terminal/input/mouseInput.cpp

@@ -321,7 +321,7 @@ bool TerminalInput::HandleMouse(const COORD position,
             // 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);
+            return IsTrackingMouseInput() || ShouldSendAlternateScroll(button, delta);
         }
 
         // We're ready to send this event through, but first we need to clear the accumulated;
@@ -330,7 +330,7 @@ bool TerminalInput::HandleMouse(const COORD position,
     }
 
     bool success = false;
-    if (_ShouldSendAlternateScroll(button, delta))
+    if (ShouldSendAlternateScroll(button, delta))
     {
         success = _SendAlternateScroll(delta);
     }
@@ -539,11 +539,12 @@ std::wstring TerminalInput::_GenerateSGRSequence(const COORD position,
 // - delta: The scroll wheel delta of the input event
 // Return value:
 // True iff the alternate buffer is active and alternate scroll mode is enabled and the event is a mouse wheel event.
-bool TerminalInput::_ShouldSendAlternateScroll(const unsigned int button, const short delta) const noexcept
+bool TerminalInput::ShouldSendAlternateScroll(const unsigned int button, const short delta) const noexcept
 {
-    return _mouseInputState.inAlternateBuffer &&
-           _inputMode.test(Mode::AlternateScroll) &&
-           (button == WM_MOUSEWHEEL || button == WM_MOUSEHWHEEL) && delta != 0;
+    const bool inAltBuffer{ _mouseInputState.inAlternateBuffer };
+    const bool inAltScroll{ _inputMode.test(Mode::AlternateScroll) };
+    const bool wasMouseWheel{ (button == WM_MOUSEWHEEL || button == WM_MOUSEHWHEEL) && delta != 0 };
+    return inAltBuffer && inAltScroll && wasMouseWheel;
 }
 
 // Routine Description:

src/terminal/input/terminalInput.hpp

@@ -73,6 +73,7 @@ namespace Microsoft::Console::VirtualTerminal
                          const MouseButtonState state);
 
         bool IsTrackingMouseInput() const noexcept;
+        bool ShouldSendAlternateScroll(const unsigned int button, const short delta) const noexcept;
 #pragma endregion
 
 #pragma region MouseInputState Management
@@ -127,7 +128,6 @@ namespace Microsoft::Console::VirtualTerminal
                                                  const short modifierKeyState,
                                                  const short delta);
 
-        bool _ShouldSendAlternateScroll(const unsigned int button, const short delta) const noexcept;
         bool _SendAlternateScroll(const short delta) const noexcept;
 
         static constexpr unsigned int s_GetPressedButton(const MouseButtonState state) noexcept;