Fix issues with Japanese & Vietnamese IME (microsoft#13678)

This commit builds directly on the changes made in microsoft#13677 and fixes:
* TSF resetting to AlphaNumeric ("ASCII") input mode when pressing enter
* Vietnamese IME not composing a new word after pressing whitespace, etc.

Closes microsoft#11479
Closes microsoft#13398

## Validation Steps Performed
* Japanese IME (Full-Width Katakana)
  Typing "saitama" produces "サイタマ" ✅
* Korean IME
  Typing "gksrmf" produces "한글" ✅
* Vietnamese IME
  Typing "xin chaof" produces "xin chào" ✅
* Emoji Picker (Win+.)
  ✅

.github/actions/spelling/expect/expect.txt

@@ -224,6 +224,7 @@ CFuzz
 cgscrn
 chafa
 changelist
+chaof
 charinfo
 charset
 CHARSETINFO
@@ -2763,6 +2764,9 @@ xes
 xff
 XFile
 XFORM
+xin
+xinchaof
+xinxinchaof
 XManifest
 XMath
 XMFLOAT

src/cascadia/TerminalControl/TSFInputControl.cpp

@@ -102,9 +102,7 @@ namespace winrt::Microsoft::Terminal::Control::implementation
             _inputBuffer.clear();
             _selection = {};
             _activeTextStart = 0;
-            _editContext.NotifyFocusLeave();
             _editContext.NotifyTextChanged({ 0, INT32_MAX }, 0, _selection);
-            _editContext.NotifyFocusEnter();
             TextBlock().Text({});
         }
     }
@@ -375,7 +373,7 @@ namespace winrt::Microsoft::Terminal::Control::implementation
                 incomingText);
             _selection = args.NewSelection();
             // GH#5054: Pressing backspace might move the caret before the _activeTextStart.
-            _activeTextStart = ::base::ClampMin(_activeTextStart, ::base::ClampedNumeric<size_t>(range.StartCaretPosition));
+            _activeTextStart = std::min(_activeTextStart, _inputBuffer.size());
 
             // Emojis/Kaomojis/Symbols chosen through the IME without starting composition
             // will be sent straight through to the terminal.

src/cascadia/TerminalControl/TermControl.cpp

@@ -932,6 +932,19 @@ namespace winrt::Microsoft::Terminal::Control::implementation
             return;
         }
 
+        // GH#11479: TSF wants to be notified of any character input via ICoreTextEditContext::NotifyTextChanged().
+        // TSF is built and tested around the idea that you inform it of any text changes that happen
+        // when it doesn't currently compose characters. For instance writing "xin chaof" with the
+        // Vietnamese IME should produce "xin chào". After writing "xin" it'll emit a composition
+        // completion event and we'll write "xin" to the shell. It now has no input focus and won't know
+        // about the whitespace. If you then write "chaof", it'll emit another composition completion
+        // event for "xinchaof" and the resulting output in the shell will finally read "xinxinchaof".
+        // A composition completion event technically doesn't mean that the completed text is now
+        // immutable after all. We could (and probably should) inform TSF of any input changes,
+        // but we technically aren't a text input field. The immediate solution was
+        // to simply force TSF to clear its text whenever we have input focus.
+        TSFInputControl().ClearBuffer();
+
         _HidePointerCursorHandlers(*this, nullptr);
 
         const auto ch = e.Character();
@@ -1193,12 +1206,6 @@ namespace winrt::Microsoft::Terminal::Control::implementation
     {
         const auto window = CoreWindow::GetForCurrentThread();
 
-        if (vkey == VK_ESCAPE ||
-            vkey == VK_RETURN)
-        {
-            TSFInputControl().ClearBuffer();
-        }
-
         // If the terminal translated the key, mark the event as handled.
         // This will prevent the system from trying to get the character out
         // of it and sending us a CharacterReceived event.