Spike: KWin input-method backend with floating layer-shell window

Registers as KWin's virtual keyboard (input-method-unstable-v1), shows a
non-focusable layer-shell overlay, and commits Unicode text. Verified in a
nested headless KWin with the textprobe tool.

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

src/window/FloatingWindow.cpp

@@ -0,0 +1,85 @@
+#include "FloatingWindow.h"
+
+#include <LayerShellQt/Window>
+#include <QGuiApplication>
+#include <QScreen>
+#include <QWindow>
+
+namespace {
+
+bool isWayland()
+{
+    return QGuiApplication::platformName().startsWith(QLatin1String("wayland"));
+}
+
+constexpr int BottomGap = 24;
+
+} // namespace
+
+FloatingWindow::FloatingWindow(QWindow *window, QObject *parent)
+    : QObject(parent)
+    , m_window(window)
+{
+    if (isWayland()) {
+        m_layerSurface = LayerShellQt::Window::get(window);
+        m_layerSurface->setLayer(LayerShellQt::Window::LayerOverlay);
+        m_layerSurface->setKeyboardInteractivity(LayerShellQt::Window::KeyboardInteractivityNone);
+        m_layerSurface->setAnchors({LayerShellQt::Window::AnchorTop | LayerShellQt::Window::AnchorLeft});
+        m_layerSurface->setExclusiveZone(-1);
+        m_layerSurface->setScope(QStringLiteral("sloposk-keyboard"));
+        m_layerSurface->setActivateOnShow(false);
+    } else {
+        window->setFlags(Qt::Tool | Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint
+                         | Qt::WindowDoesNotAcceptFocus);
+    }
+    m_position = defaultPosition();
+    apply();
+}
+
+void FloatingWindow::setPosition(QPoint position)
+{
+    position = clamped(position);
+    if (position == m_position)
+        return;
+    m_position = position;
+    apply();
+    Q_EMIT positionChanged();
+}
+
+void FloatingWindow::moveBy(qreal dx, qreal dy)
+{
+    setPosition(m_position + QPoint(qRound(dx), qRound(dy)));
+}
+
+void FloatingWindow::show()
+{
+    m_window->setVisible(true);
+}
+
+void FloatingWindow::hide()
+{
+    m_window->setVisible(false);
+}
+
+QPoint FloatingWindow::clamped(QPoint position) const
+{
+    const QRect area = m_window->screen()->geometry();
+    const int maxX = qMax(0, area.width() - m_window->width());
+    const int maxY = qMax(0, area.height() - m_window->height());
+    return {qBound(0, position.x(), maxX), qBound(0, position.y(), maxY)};
+}
+
+QPoint FloatingWindow::defaultPosition() const
+{
+    const QRect area = m_window->screen()->geometry();
+    return clamped({(area.width() - m_window->width()) / 2,
+                    area.height() - m_window->height() - BottomGap});
+}
+
+void FloatingWindow::apply()
+{
+    if (m_layerSurface)
+        m_layerSurface->setMargins({m_position.x(), m_position.y(), 0, 0});
+    else
+        m_window->setPosition(m_window->screen()->geometry().topLeft() + m_position);
+}