Add support for alternate scroll mode in Terminal

_⚠️ targets #12561 ⚠️_

"Alternate scroll mode" is a neat little mode where the app wants mouse wheel events to come through as arrow keypresses instead, when in the alternate buffer. Now that we've got support for the alt buffer in the Terminal, we can support this as well.

* [x] Closes #3321
* [x] I work here
* [ ] Tests would be nice

Tested manually with

```bash
printf "\e[?1007h" ; man ps
```

src/cascadia/TerminalControl/ControlCore.cpp

@@ -1432,6 +1432,11 @@ 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
+        // seperately 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

@@ -543,6 +543,20 @@ 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

@@ -152,6 +152,7 @@ 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

@@ -621,6 +621,10 @@ void Terminal::UseAlternateScreenBuffer()
     // update all the hyperlinks on the screen
     UpdatePatternsUnderLock();
 
+    // 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();
 
@@ -661,6 +665,10 @@ void Terminal::UseMainScreenBuffer()
     // update all the hyperlinks on the screen
     UpdatePatternsUnderLock();
 
+    // 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

@@ -324,7 +324,7 @@ 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;
@@ -333,7 +333,7 @@ bool TerminalInput::HandleMouse(const COORD position,
     }
 
     bool success = false;
-    if (_ShouldSendAlternateScroll(button, delta))
+    if (ShouldSendAlternateScroll(button, delta))
     {
         success = _SendAlternateScroll(delta);
     }
@@ -542,11 +542,12 @@ 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;