wpf: only dismiss selection for real chars, not modifiers (#5388)

Selection would act up when you were using shift to ignore VT mouse
mode: we would get hundreds of WM_KEYDOWN for VK_SHIFT and dismiss the
selection every time.

I took the opportunity to move the actual responsibility for key event
dispatch into HwndTerminal. In the future, I'd like to make more of the
TerminalXxx calls just call impl methods on HwndTerminal.

src/cascadia/PublicTerminalCore/HwndTerminal.cpp

@@ -555,23 +555,49 @@ catch (...)
     return false;
 }
 
-void _stdcall TerminalSendKeyEvent(void* terminal, WORD vkey, WORD scanCode)
+void HwndTerminal::_SendKeyEvent(WORD vkey, WORD scanCode) noexcept
+try
 {
-    const auto publicTerminal = static_cast<const HwndTerminal*>(terminal);
     const auto flags = getControlKeyState();
-    publicTerminal->_terminal->SendKeyEvent(vkey, scanCode, flags);
+    _terminal->SendKeyEvent(vkey, scanCode, flags);
 }
+CATCH_LOG();
 
-void _stdcall TerminalSendCharEvent(void* terminal, wchar_t ch, WORD scanCode)
+void HwndTerminal::_SendCharEvent(wchar_t ch, WORD scanCode) noexcept
+try
 {
-    if (ch == '\t')
+    if (_terminal->IsSelectionActive())
     {
+        _terminal->ClearSelection();
+        if (ch == UNICODE_ESC)
+        {
+            // ESC should clear any selection before it triggers input.
+            // Other characters pass through.
+            return;
+        }
+    }
+
+    if (ch == UNICODE_TAB)
+    {
+        // TAB was handled as a keydown event (cf. Terminal::SendKeyEvent)
         return;
     }
 
-    const auto publicTerminal = static_cast<const HwndTerminal*>(terminal);
     const auto flags = getControlKeyState();
-    publicTerminal->_terminal->SendCharEvent(ch, scanCode, flags);
+    _terminal->SendCharEvent(ch, scanCode, flags);
+}
+CATCH_LOG();
+
+void _stdcall TerminalSendKeyEvent(void* terminal, WORD vkey, WORD scanCode)
+{
+    const auto publicTerminal = static_cast<HwndTerminal*>(terminal);
+    publicTerminal->_SendKeyEvent(vkey, scanCode);
+}
+
+void _stdcall TerminalSendCharEvent(void* terminal, wchar_t ch, WORD scanCode)
+{
+    const auto publicTerminal = static_cast<HwndTerminal*>(terminal);
+    publicTerminal->_SendCharEvent(ch, scanCode);
 }
 
 void _stdcall DestroyTerminal(void* terminal)

src/cascadia/PublicTerminalCore/HwndTerminal.hpp

@@ -115,6 +115,9 @@ struct HwndTerminal : ::Microsoft::Console::Types::IControlAccessibilityInfo
     bool _CanSendVTMouseInput() const noexcept;
     bool _SendMouseEvent(UINT uMsg, WPARAM wParam, LPARAM lParam) noexcept;
 
+    void _SendKeyEvent(WORD vkey, WORD scanCode) noexcept;
+    void _SendCharEvent(wchar_t ch, WORD scanCode) noexcept;
+
     // Inherited via IControlAccessibilityInfo
     COORD GetFontSize() const override;
     RECT GetBounds() const noexcept override;

src/cascadia/WpfTerminalControl/TerminalContainer.cs

@@ -235,7 +235,6 @@ private IntPtr TerminalContainer_MessageHook(IntPtr hwnd, int msg, IntPtr wParam
                     case NativeMethods.WindowMessage.WM_KEYDOWN:
                         // WM_KEYDOWN lParam layout documentation: https://docs.microsoft.com/en-us/windows/win32/inputdev/wm-keydown
                         NativeMethods.TerminalSetCursorVisible(this.terminal, true);
-                        NativeMethods.TerminalClearSelection(this.terminal);
                         NativeMethods.TerminalSendKeyEvent(this.terminal, (ushort)wParam, (ushort)((uint)lParam >> 16));
                         this.blinkTimer?.Start();
                         break;