Keep the user's shift; trust the text guess less

- A tapped shift (on or off) survives the field re-reporting its context:
  it lasts until a character is typed, the hint changes or a field is focused
- An input-method reset forgets the guessed text and pending dead key, so
  double-space can't delete a character the keyboard never saw
- Fields that state typing preferences only get capitals if they ask
- Trackpad moves in fields without surrounding text make the guess unknown;
  Backspace removes a whole grapheme cluster from it

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/input/KeyboardController.cpp

@@ -14,6 +14,9 @@ static_assert(int(KeyboardController::ShiftMode::Locked) == int(ShiftState::Mode
 namespace {
 // Enough history for the text rules; fields may report megabytes.
 constexpr qsizetype MaxTextBefore = 256;
+// Stands for text we can't see: neither a sentence start nor a word, so no
+// rule fires on it. For fields that don't report text, after cursor moves.
+constexpr QChar UnknownText = u'\uFFFC';
 // Like a hardware key: no word mode, a faster rate than Backspace.
 constexpr KeyRepeater::Timing KeysymRepeat{.intervalMs = 50, .wordModeAfterMs = std::numeric_limits<int>::max()};
 }
@@ -168,20 +171,30 @@ void KeyboardController::moveCursor(QPoint steps)
     repeat(qMax(0, -steps.x()), XKB_KEY_Left);
     repeat(qMax(0, steps.y()), XKB_KEY_Down);
     repeat(qMax(0, -steps.y()), XKB_KEY_Up);
+    if (!steps.isNull() && !m_context.hasSurroundingText) {
+        m_textBefore = UnknownText;
+        autoCapitalize();
+    }
 }
 
 void KeyboardController::resetContext(const TextContext &context)
 {
     m_deadKeys.cancel();
     m_textBefore.clear();
+    m_shift.forgetUserChoice();
     setContext(context);
 }
 
 void KeyboardController::setContext(const TextContext &context)
 {
-    m_context = context;
-    if (context.hasSurroundingText)
+    if (context.hasSurroundingText) {
         m_textBefore = context.textBeforeCursor.right(MaxTextBefore);
+    } else if (m_context.hasSurroundingText) {
+        // The field stopped reporting (an input-method reset): what we knew is stale.
+        m_deadKeys.cancel();
+        m_textBefore.clear();
+    }
+    m_context = context;
     autoCapitalize();
 }