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diff --git a/src/tagdialog.h b/src/tagdialog.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 <QDialog>
+#include <QStringList>
+
+class QLineEdit;
+class QListWidget;
+
+/// Why a tag name was refused, or Ok when it was not.
+///
+/// A reason rather than a bool: a rejected tag has to say what was wrong with
+/// it, since silently dropping one leaves the user believing they tagged
+/// something they did not.
+enum class TagNameProblem {
+    Ok,
+    Empty,          ///< Nothing, or only whitespace.
+    LeadingDash,    ///< notmuch reads a leading '-' as "remove this tag".
+    ContainsSpace,  ///< Splits into two tags, or fails the write outright.
+    ControlChar,    ///< Not typeable, and unreadable once stored.
+};
+
+/// Whether `tag` is safe to hand to notmuch.
+///
+/// A free function so the rules can be tested without a widget. notmuch itself
+/// accepts a great deal, so this is deliberately narrow: it rejects only what
+/// produces a failed write or a tag the user cannot see they created.
+TagNameProblem validateTagName(const QString &tag);
+
+/// The message for a rejected tag, ready to show. Empty for Ok.
+QString tagNameProblemText(TagNameProblem problem, const QString &tag);
+
+/// Adds and removes tags across the selected threads.
+///
+/// One dialog rather than separate add and remove actions: the natural
+/// operation is "make these threads look like this", and filing something under
+/// a new tag while removing inbox is one thought, not two.
+///
+/// Pure UI. It contacts no worker and holds no database handle; it is handed
+/// the vocabulary and the current state, and returns two lists. That is what
+/// lets it be unit-tested without a notmuch database.
+class TagDialog : public QDialog
+{
+    Q_OBJECT
+public:
+    /// `knownTags` is the completion vocabulary, usually every tag in the
+    /// database. `currentTags` maps a tag to how many of the selected threads
+    /// carry it, which is what drives the tri-state checkboxes.
+    TagDialog(const QStringList &knownTags,
+              const QHash<QString, int> &currentTags,
+              int threadCount,
+              QWidget *parent = nullptr);
+
+    /// Tags to add. Empty when the user asked for nothing.
+    QStringList tagsToAdd() const;
+
+    /// Tags to remove.
+    QStringList tagsToRemove() const;
+
+    /// Reads both line edits and the checkbox list into the two lists,
+    /// reporting the first invalid name rather than applying a partial change.
+    ///
+    /// Public because QDialog::accept() is: a test drives it directly rather
+    /// than clicking a button, since which button carries the AcceptRole is
+    /// not what this class is for.
+    void accept() override;
+
+private:
+
+    QStringList m_add;
+    QStringList m_remove;
+
+    /// Tags that were already on EVERY selected thread when the dialog opened.
+    ///
+    /// Needed to tell "the user checked this box" apart from "this box was
+    /// checked all along": the first is an instruction, the second is not a
+    /// change and must not be sent as one.
+    QStringList m_fullyTagged;
+
+    int m_threadCount = 0;
+
+    QLineEdit *m_addEdit = nullptr;
+    QLineEdit *m_removeEdit = nullptr;
+    QListWidget *m_currentList = nullptr;
+};