ControllerInterface: Add platform consistent names for modifier keys.

Source/Core/InputCommon/ControllerInterface/DInput/DInputKeyboardMouse.cpp

@@ -87,6 +87,11 @@ KeyboardMouse::KeyboardMouse(const LPDIRECTINPUTDEVICE8 kb_device,
   for (u8 i = 0; i < sizeof(named_keys) / sizeof(*named_keys); ++i)
     AddInput(new Key(i, m_state_in.keyboard[named_keys[i].code]));
 
+  // Add combined left/right modifiers with consistent naming across platforms.
+  AddCombinedInput("Alt", {"LMENU", "RMENU"});
+  AddCombinedInput("Shift", {"LSHIFT", "RSHIFT"});
+  AddCombinedInput("Ctrl", {"LCONTROL", "RCONTROL"});
+
   // MOUSE
   DIDEVCAPS mouse_caps = {};
   mouse_caps.dwSize = sizeof(mouse_caps);

Source/Core/InputCommon/ControllerInterface/Device.cpp

@@ -103,6 +103,39 @@ bool Device::FullAnalogSurface::IsMatchingName(std::string_view name) const
   return old_name == name;
 }
 
+Device::CombinedInput::CombinedInput(std::string name, const Inputs& inputs)
+    : m_name(std::move(name)), m_inputs(inputs)
+{
+}
+
+ControlState Device::CombinedInput::GetState() const
+{
+  ControlState result = 0;
+
+  if (m_inputs.first)
+    result = m_inputs.first->GetState();
+
+  if (m_inputs.second)
+    result = std::max(result, m_inputs.second->GetState());
+
+  return result;
+}
+
+std::string Device::CombinedInput::GetName() const
+{
+  return m_name;
+}
+
+bool Device::CombinedInput::IsDetectable()
+{
+  return false;
+}
+
+void Device::AddCombinedInput(std::string name, const std::pair<std::string, std::string>& inputs)
+{
+  AddInput(new CombinedInput(std::move(name), {FindInput(inputs.first), FindInput(inputs.second)}));
+}
+
 //
 // DeviceQualifier :: ToString
 //

Source/Core/InputCommon/ControllerInterface/Device.h

@@ -147,6 +147,23 @@ protected:
     AddInput(new FullAnalogSurface(high, low));
   }
 
+  class CombinedInput final : public Input
+  {
+  public:
+    using Inputs = std::pair<Input*, Input*>;
+
+    CombinedInput(std::string name, const Inputs& inputs);
+    ControlState GetState() const override;
+    std::string GetName() const override;
+    bool IsDetectable() override;
+
+  private:
+    const std::string m_name;
+    const std::pair<Input*, Input*> m_inputs;
+  };
+
+  void AddCombinedInput(std::string name, const std::pair<std::string, std::string>& inputs);
+
 private:
   int m_id;
   std::vector<Input*> m_inputs;

Source/Core/InputCommon/ControllerInterface/Quartz/QuartzKeyboardAndMouse.mm

@@ -143,6 +143,11 @@ KeyboardAndMouse::KeyboardAndMouse(void* window)
   for (int keycode = 0; keycode < 0x80; ++keycode)
     AddInput(new Key(keycode));
 
+  // Add combined left/right modifiers with consistent naming across platforms.
+  AddCombinedInput("Alt", {"Left Alt", "Right Alt"});
+  AddCombinedInput("Shift", {"Left Shift", "Right Shift"});
+  AddCombinedInput("Ctrl", {"Left Control", "Right Control"});
+
   m_windowid = [[reinterpret_cast<NSView*>(window) window] windowNumber];
 
   // cursor, with a hax for-loop

Source/Core/InputCommon/ControllerInterface/Xlib/XInput2.cpp

@@ -172,6 +172,11 @@ KeyboardMouse::KeyboardMouse(Window window, int opcode, int pointer, int keyboar
       delete temp_key;
   }
 
+  // Add combined left/right modifiers with consistent naming across platforms.
+  AddCombinedInput("Alt", {"Alt_L", "Alt_R"});
+  AddCombinedInput("Shift", {"Shift_L", "Shift_R"});
+  AddCombinedInput("Ctrl", {"Control_L", "Control_R"});
+
   // Mouse Buttons
   for (int i = 0; i < 32; i++)
     AddInput(new Button(i, &m_state.buttons));