Text rules: auto-capitalization, double-space period, backspace repeat

- TextContext: field purpose/hints and text before the cursor, reported by
  the backend (KWin content_type + surrounding_text)
- TextRules: sentence-start and double-space heuristics, deletion helpers
- KeyRepeater: held backspace repeats, then deletes whole words (Ctrl+BS)
- Controller keeps a best-guess of the text before the cursor so rules work
  instantly and in fields that don't report surrounding text
- No auto-capitalization or smart punctuation in password/URL/email fields

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

src/input/KeyboardController.cpp

@@ -1,4 +1,5 @@
 #include "KeyboardController.h"
+#include "TextRules.h"
 
 #include "engine/InputEngine.h"
 #include "layout/LayoutModel.h"
@@ -11,6 +12,11 @@ static_assert(int(KeyboardController::ShiftMode::Off) == int(ShiftState::Mode::O
 static_assert(int(KeyboardController::ShiftMode::Once) == int(ShiftState::Mode::Once));
 static_assert(int(KeyboardController::ShiftMode::Locked) == int(ShiftState::Mode::Locked));
 
+namespace {
+// Enough history for the text rules; fields may report megabytes.
+constexpr qsizetype MaxTextBefore = 256;
+}
+
 KeyboardController::KeyboardController(InputEngine &engine, LayoutModel &model,
                                        const LayoutRepository &layouts, QObject *parent)
     : QObject(parent)
@@ -20,6 +26,7 @@ KeyboardController::KeyboardController(InputEngine &engine, LayoutModel &model,
 {
     if (!m_layouts.layouts().isEmpty())
         m_model.setLayout(m_layouts.layouts().first());
+    connect(&m_backspaceRepeater, &KeyRepeater::repeat, this, &KeyboardController::deleteBackward);
 }
 
 KeyboardController::ShiftMode KeyboardController::shiftMode() const
@@ -34,7 +41,8 @@ void KeyboardController::keyDown(const KeyDef &key, qint64 timeMs)
         changeShift([&] { m_shift.press(timeMs); });
         break;
     case KeyType::Backspace:
-        m_engine.sendKeysym(XKB_KEY_BackSpace);
+        deleteBackward(false);
+        m_backspaceRepeater.start();
         break;
     case KeyType::LayerSwitch:
         m_layerBeforeSwitch = m_model.layerName();
@@ -51,6 +59,9 @@ void KeyboardController::keyUp(const KeyDef &key, KeyOutput output, const QStrin
     case KeyType::Shift:
         changeShift([&] { m_shift.release(); });
         break;
+    case KeyType::Backspace:
+        m_backspaceRepeater.stop();
+        break;
     case KeyType::Character:
         if (output == KeyOutput::Alternate && !alternate.isEmpty())
             typeText(alternate);
@@ -64,6 +75,8 @@ void KeyboardController::keyUp(const KeyDef &key, KeyOutput output, const QStrin
         break;
     case KeyType::Enter:
         m_engine.sendKeysym(XKB_KEY_Return);
+        m_textBefore += u'\n';
+        autoCapitalize();
         break;
     case KeyType::Globe:
         nextLayout();
@@ -80,6 +93,8 @@ void KeyboardController::keyCancelled(const KeyDef &key)
 {
     if (key.type == KeyType::Shift)
         changeShift([&] { m_shift.release(); });
+    else if (key.type == KeyType::Backspace)
+        m_backspaceRepeater.stop();
 }
 
 void KeyboardController::slideReleased(const KeyDef &target, qint64 timeMs)
@@ -100,6 +115,21 @@ void KeyboardController::moveCursor(QPoint steps)
     repeat(qMax(0, -steps.y()), XKB_KEY_Up);
 }
 
+void KeyboardController::resetContext()
+{
+    m_context = {};
+    m_textBefore.clear();
+    autoCapitalize();
+}
+
+void KeyboardController::setContext(const TextContext &context)
+{
+    m_context = context;
+    if (context.hasSurroundingText)
+        m_textBefore = context.textBeforeCursor.right(MaxTextBefore);
+    autoCapitalize();
+}
+
 void KeyboardController::nextLayout()
 {
     const auto &layouts = m_layouts.layouts();
@@ -111,8 +141,31 @@ void KeyboardController::nextLayout()
 
 void KeyboardController::typeText(const QString &text)
 {
+    if (text == u" " && m_context.isProse() && TextRules::wantsDoubleSpacePeriod(m_textBefore)) {
+        m_engine.sendKeysym(XKB_KEY_BackSpace);
+        m_textBefore.chop(1);
+        typeText(QStringLiteral(". "));
+        return;
+    }
+
     m_engine.commitText(text);
+    m_textBefore = (m_textBefore + text).right(MaxTextBefore);
     changeShift([&] { m_shift.characterTyped(); });
+    autoCapitalize();
+}
+
+void KeyboardController::deleteBackward(bool wholeWord)
+{
+    m_engine.sendKeysym(XKB_KEY_BackSpace, wholeWord ? Qt::ControlModifier : Qt::NoModifier);
+    m_textBefore = wholeWord ? TextRules::withoutLastWord(m_textBefore)
+                             : TextRules::withoutLastCharacter(m_textBefore);
+    autoCapitalize();
+}
+
+void KeyboardController::autoCapitalize()
+{
+    const bool capitalize = m_context.allowsAutoCapitalization() && TextRules::startsSentence(m_textBefore);
+    changeShift([&] { m_shift.setAutomatic(capitalize); });
 }
 
 template<typename Change>