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diff --git a/src/calendarwindow.h b/src/calendarwindow.h
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+/*
+ * qtmaildir - a Qt6 mail client for notmuch-indexed Maildirs
+ * Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#pragma once
+
+#include "caltypes.h"
+#include "config.h"
+
+#include <QFileSystemWatcher>
+#include <QHash>
+#include <QMainWindow>
+#include <QTimer>
+#include <QUndoStack>
+
+#include <optional>
+
+class AgendaView;
+class CalendarSync;
+class EventPane;
+class MonthView;
+class QComboBox;
+class QLabel;
+class QSpinBox;
+class QStackedWidget;
+class QToolButton;
+
+/// The calendar (item 206): its own top-level window, like ComposeWindow.
+/// Spec: docs/superpowers/specs/2026-09-24-calendar-design.md.
+class CalendarWindow : public QMainWindow
+{
+    Q_OBJECT
+
+public:
+    CalendarWindow(const Config &config, const QStringList &ownAddresses,
+                   const QString &uiStatePath, QWidget *parent = nullptr);
+    ~CalendarWindow() override;
+
+    void showMonth(int year, int month);
+    /// Selects an event's occurrence (the first in view when `start` is
+    /// invalid). False when not found, or refused by the edit lock.
+    bool selectEvent(const QString &uid, const QDateTime &start = {});
+    QString selectedUid() const;
+    bool isEditing() const;
+    QUndoStack *undoStack() { return &m_undo; }
+
+    /// One file's before and after, either absent. What a write and its undo
+    /// are made of.
+    struct Change
+    {
+        QString path;
+        std::optional<QByteArray> before;
+        std::optional<QByteArray> after;
+    };
+    /// Applies `changes` in order through CalendarWriter. On a failure the
+    /// ones already applied are rolled back and the reason is reported.
+    bool applyChanges(const QList<Change> &changes, bool reverse);
+
+protected:
+    void closeEvent(QCloseEvent *event) override;
+
+private:
+    void buildUi();
+    void buildActions();
+    void reload();
+    void rebuildItems();
+    void refreshToolbar();
+    void select(int item);
+    void startEdit();
+    void startNew(const QDate &day);
+    QDate defaultNewDay() const;
+    bool save();
+    void cancelEdit();
+    void deleteSelected();
+    bool writeAndRecord(const QString &label, const QList<Change> &changes);
+    void checkAfterSync();
+    const CalCollection *collection(const QString &dir) const;
+    QList<CalCollection> writableCollections() const;
+    QString defaultCollection() const;
+    void status(const QString &text);
+
+    Config m_config;
+    QStringList m_ownAddresses;
+    QString m_uiStatePath;
+
+    LoadResult m_data;
+    QList<CalendarItem> m_items;
+    int m_year;
+    int m_month;
+    int m_selected = -1;
+    QString m_selectedUid;         ///< survives a reload
+    QDateTime m_selectedStart;
+
+    // The open edit. m_editPath empty with m_editing: a new event.
+    QString m_editPath;
+    QByteArray m_editBase;
+    bool m_editWholeSeries = true;
+    QDateTime m_editRecurrenceId;
+
+    QUndoStack m_undo;
+    CalendarSync *m_sync;
+    QFileSystemWatcher m_watcher;
+    QTimer m_reloadTimer;
+    /// Written since the last sync started: path to the text written, or
+    /// nullopt for a deletion. Checked by meaning after the next sync.
+    QHash<QString, std::optional<QByteArray>> m_written;
+    QHash<QString, std::optional<QByteArray>> m_checking;
+
+    QComboBox *m_collectionBox;
+    QComboBox *m_monthBox;
+    QSpinBox *m_yearSpin;
+    QToolButton *m_monthButton;
+    QToolButton *m_agendaButton;
+    QStackedWidget *m_views;
+    MonthView *m_monthView;
+    AgendaView *m_agendaView;
+    EventPane *m_pane;
+    QLabel *m_dataWarning;
+};