Initial WindowGrid KWin effect for Plasma 6

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

windowgrid.h

@@ -0,0 +1,91 @@
+/*
+    SPDX-License-Identifier: GPL-3.0-or-later
+*/
+
+#pragma once
+
+#include "effect/effect.h"
+
+#include <QColor>
+#include <QMetaObject>
+#include <QPointer>
+#include <QRectF>
+
+namespace KWin
+{
+
+class EffectWindow;
+class LogicalOutput;
+
+class WindowGridEffect : public Effect
+{
+    Q_OBJECT
+
+public:
+    WindowGridEffect();
+    ~WindowGridEffect() override;
+
+    void reconfigure(ReconfigureFlags flags) override;
+    bool isActive() const override;
+    bool paintScreen(const RenderTarget &renderTarget, const RenderViewport &viewport, int mask, const Region &deviceRegion, LogicalOutput *screen) override;
+    void pointerMotion(PointerMotionEvent *event) override;
+    void pointerButton(PointerButtonEvent *event) override;
+    void grabbedKeyboardEvent(QKeyEvent *e) override;
+
+private:
+    // Native: KWin is doing the normal title bar move. Grid: we own the pointer and
+    // show the overlay. Moving: middle button held, window follows the cursor.
+    enum class State {
+        Idle,
+        Native,
+        Grid,
+        Moving,
+    };
+
+    struct Cell
+    {
+        int col = 0;
+        int row = 0;
+    };
+
+    void watchWindow(EffectWindow *w);
+    void onMoveStarted(EffectWindow *w);
+    void onMoveFinished(EffectWindow *w);
+    void onMouseChanged(const QPointF &pos, const QPointF &oldpos, Qt::MouseButtons buttons, Qt::MouseButtons oldbuttons);
+
+    void enterGrid(bool viaSpace);
+    void finish(bool restoreGeometry);
+    void updateArea(const QPointF &pos);
+    Cell cellAt(const QPointF &pos) const;
+    QRectF cellRect(Cell a, Cell b) const;
+    void setSelecting(bool rightButton, bool space, const QPointF &pos);
+    void applySelection();
+    void moveWithCursor(const QPointF &pos);
+
+    State m_state = State::Idle;
+    QPointer<EffectWindow> m_window;
+    QMetaObject::Connection m_mouseConnection;
+    bool m_keyboardGrabbed = false;
+
+    QRectF m_originalGeometry;
+    LogicalOutput *m_screen = nullptr;
+    QRectF m_area;
+    int m_cols = 1;
+    int m_rows = 1;
+    Cell m_start;
+    Cell m_end;
+    bool m_rightHeld = false;
+    bool m_spaceHeld = false;
+    QPointF m_grabOffset;
+    QPointF m_cursor;
+
+    // config, group "Effect-windowgrid" in kwinrc
+    int m_cfgColumns = 6;
+    int m_cfgRows = 4;
+    bool m_useWorkArea = true;
+    QColor m_gridColor;
+    QColor m_selectionColor;
+    qreal m_opacity = 0.35;
+};
+
+} // namespace KWin