Slide sideways on space to move the cursor

As on phone keyboards: a sideways move on space moves the cursor right
away, along the line only, without blanking the keys. Long-press space
still opens the full trackpad.

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

tests/tst_keyboardsurface.cpp

@@ -208,6 +208,17 @@ private Q_SLOTS:
         QCOMPARE(engine.output, QString());
         QCOMPARE(engine.keysyms, (QList<quint32>{XKB_KEY_Right, XKB_KEY_Right, XKB_KEY_Right}));
     }
+
+    void spaceSlideMovesCursorAtOnce()
+    {
+        const QPoint space = keyCenter({}, u"space"_s);
+        QTest::touchEvent(&window, touchscreen).press(0, space);
+        QTest::touchEvent(&window, touchscreen).move(0, space - QPoint(30, 0));
+        QVERIFY(!qobject_cast<KeyboardSurface *>(keyboard)->isTrackpadActive()); // keys keep their labels
+        QTest::touchEvent(&window, touchscreen).release(0, space - QPoint(30, 0));
+        QCOMPARE(engine.output, QString());
+        QCOMPARE(engine.keysyms, (QList<quint32>{XKB_KEY_Left, XKB_KEY_Left}));
+    }
 };
 
 QTEST_MAIN(TestKeyboardSurface)

tests/tst_pointertracker.cpp

@@ -111,6 +111,33 @@ private Q_SLOTS:
         QCOMPARE(t.takeTrackpadSteps(), QPoint(-3, -1));
     }
 
+    void spaceSlideSidewaysMovesAlongTheLine()
+    {
+        PointerTracker t(ofType(KeyType::Space), {100, 0}, config());
+        t.moveTo({112, 5});
+        QCOMPARE(t.gesture(), Gesture::Trackpad);
+        QCOMPARE(t.takeTrackpadSteps(), QPoint(1, 0)); // the slop counts
+        t.moveTo({75, -50});
+        QCOMPARE(t.takeTrackpadSteps(), QPoint(-3, 0)); // no line changes
+        QVERIFY(!t.longPressElapsed());
+    }
+
+    void spaceSlideVerticallyStaysTap()
+    {
+        PointerTracker t(ofType(KeyType::Space), {100, 0}, config());
+        t.moveTo({105, -20});
+        QCOMPARE(t.gesture(), Gesture::Tap);
+    }
+
+    void spaceSlideNeedsGestures()
+    {
+        PointerTracker::Config c = config();
+        c.gestures = false;
+        PointerTracker t(ofType(KeyType::Space), {100, 0}, c);
+        t.moveTo({140, 0});
+        QCOMPARE(t.gesture(), Gesture::Tap);
+    }
+
     void layerKeySlides()
     {
         PointerTracker t(ofType(KeyType::LayerSwitch), {0, 0}, config());