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diff --git a/tests/test_signatures.cpp b/tests/test_signatures.cpp
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+++ b/tests/test_signatures.cpp
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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.
+ */
+
+#include <QtTest>
+#include <QTemporaryDir>
+
+#include "signatures.h"
+
+class TestSignatures : public QObject
+{
+    Q_OBJECT
+
+private slots:
+    void namesAreTheFileStemsSorted();
+    void namesIgnoreFilesThatAreNotMarkdown();
+    void aMissingDirectoryHasNoNames();
+    void textIsTheFileContent();
+    void textOfAnUnknownNameIsEmpty();
+    void insertingAtTheEndAppendsAfterADelimiter();
+    void insertingAboveTheQuotePutsItBeforeTheFirstQuotedLine();
+    void insertingAboveTheQuoteWithNoQuoteIsTheSameAsEnd();
+    void insertingNothingLeavesTheBufferAlone();
+    void switchingReplacesAKnownSignature();
+    void switchingReplacesAKnownSignatureAboveAQuote();
+    void selectingNoneRemovesAKnownSignature();
+    void aBlockMatchingNoKnownSignatureIsNotRemoved();
+    void aDelimiterInsideTheQuoteIsNotTheSignature();
+    void aSignatureReadBackFromDiskIsReplaced();
+
+private:
+    /// Writes \p files as name -> content into a fresh temporary directory.
+    static void write(const QTemporaryDir &dir,
+                      const QList<QPair<QString, QString>> &files);
+};
+
+void TestSignatures::write(const QTemporaryDir &dir,
+                           const QList<QPair<QString, QString>> &files)
+{
+    for (const auto &entry : files) {
+        QFile file(dir.path() + QStringLiteral("/") + entry.first);
+        QVERIFY(file.open(QIODevice::WriteOnly | QIODevice::Text));
+        file.write(entry.second.toUtf8());
+        file.close();
+    }
+}
+
+void TestSignatures::namesAreTheFileStemsSorted()
+{
+    QTemporaryDir dir;
+    QVERIFY(dir.isValid());
+    write(dir, { { QStringLiteral("work.md"), QStringLiteral("Work") },
+                 { QStringLiteral("brief.md"), QStringLiteral("Brief") } });
+
+    QCOMPARE(Signatures::names(dir.path()),
+             QStringList({ QStringLiteral("brief"), QStringLiteral("work") }));
+}
+
+void TestSignatures::namesIgnoreFilesThatAreNotMarkdown()
+{
+    QTemporaryDir dir;
+    QVERIFY(dir.isValid());
+    write(dir, { { QStringLiteral("work.md"), QStringLiteral("Work") },
+                 { QStringLiteral("notes.txt"), QStringLiteral("Not one") },
+                 { QStringLiteral("README"), QStringLiteral("Nor this") } });
+
+    QCOMPARE(Signatures::names(dir.path()),
+             QStringList({ QStringLiteral("work") }));
+}
+
+void TestSignatures::aMissingDirectoryHasNoNames()
+{
+    QTemporaryDir dir;
+    QVERIFY(dir.isValid());
+    const QString missing = dir.path() + QStringLiteral("/nothing-here");
+
+    QVERIFY(Signatures::names(missing).isEmpty());
+}
+
+void TestSignatures::textIsTheFileContent()
+{
+    QTemporaryDir dir;
+    QVERIFY(dir.isValid());
+    write(dir, { { QStringLiteral("work.md"),
+                   QStringLiteral("Jane Doe\n**qtmaildir**\n") } });
+
+    QCOMPARE(Signatures::text(dir.path(), QStringLiteral("work")),
+             QStringLiteral("Jane Doe\n**qtmaildir**\n"));
+}
+
+void TestSignatures::textOfAnUnknownNameIsEmpty()
+{
+    QTemporaryDir dir;
+    QVERIFY(dir.isValid());
+
+    QVERIFY(Signatures::text(dir.path(), QStringLiteral("absent")).isEmpty());
+}
+
+void TestSignatures::insertingAtTheEndAppendsAfterADelimiter()
+{
+    const QString buffer = QStringLiteral("Hello.\n");
+
+    const QString result = Signatures::replace(
+        buffer, QStringLiteral("Jane Doe"), {}, Signatures::Position::End);
+
+    QCOMPARE(result, QStringLiteral("Hello.\n\n-- \nJane Doe"));
+}
+
+void TestSignatures::insertingAboveTheQuotePutsItBeforeTheFirstQuotedLine()
+{
+    const QString buffer = QStringLiteral(
+        "My reply.\n"
+        "\n"
+        "On Mon, someone wrote:\n"
+        "> the original\n"
+        "> second line\n");
+
+    const QString result = Signatures::replace(
+        buffer, QStringLiteral("Jane Doe"), {},
+        Signatures::Position::AboveQuote);
+
+    // Before the QUOTED lines, and the attribution stays with the quote it
+    // introduces: it is the line the quote hangs from, not part of the reply.
+    QCOMPARE(result, QStringLiteral(
+        "My reply.\n"
+        "\n"
+        "-- \n"
+        "Jane Doe\n"
+        "\n"
+        "On Mon, someone wrote:\n"
+        "> the original\n"
+        "> second line\n"));
+}
+
+void TestSignatures::insertingAboveTheQuoteWithNoQuoteIsTheSameAsEnd()
+{
+    const QString buffer = QStringLiteral("A new message.\n");
+
+    const QString above = Signatures::replace(
+        buffer, QStringLiteral("Jane Doe"), {},
+        Signatures::Position::AboveQuote);
+    const QString end = Signatures::replace(
+        buffer, QStringLiteral("Jane Doe"), {}, Signatures::Position::End);
+
+    QCOMPARE(above, end);
+}
+
+void TestSignatures::insertingNothingLeavesTheBufferAlone()
+{
+    const QString buffer = QStringLiteral("Hello.\n");
+
+    QCOMPARE(Signatures::replace(buffer, QString(), {},
+                                 Signatures::Position::End),
+             buffer);
+}
+
+void TestSignatures::switchingReplacesAKnownSignature()
+{
+    const QStringList known = { QStringLiteral("Jane Doe"),
+                                QStringLiteral("Jane Doe\nqtmaildir") };
+    const QString buffer = QStringLiteral("Hello.\n\n-- \nJane Doe");
+
+    const QString result = Signatures::replace(
+        buffer, QStringLiteral("Jane Doe\nqtmaildir"), known,
+        Signatures::Position::End);
+
+    QCOMPARE(result,
+             QStringLiteral("Hello.\n\n-- \nJane Doe\nqtmaildir"));
+}
+
+void TestSignatures::switchingReplacesAKnownSignatureAboveAQuote()
+{
+    const QStringList known = { QStringLiteral("Jane Doe"),
+                                QStringLiteral("Brief") };
+    const QString buffer = QStringLiteral(
+        "My reply.\n"
+        "\n"
+        "-- \n"
+        "Jane Doe\n"
+        "\n"
+        "On Mon, someone wrote:\n"
+        "> the original\n");
+
+    const QString result = Signatures::replace(
+        buffer, QStringLiteral("Brief"), known,
+        Signatures::Position::AboveQuote);
+
+    QCOMPARE(result, QStringLiteral(
+        "My reply.\n"
+        "\n"
+        "-- \n"
+        "Brief\n"
+        "\n"
+        "On Mon, someone wrote:\n"
+        "> the original\n"));
+}
+
+void TestSignatures::selectingNoneRemovesAKnownSignature()
+{
+    const QStringList known = { QStringLiteral("Jane Doe") };
+    const QString buffer = QStringLiteral("Hello.\n\n-- \nJane Doe");
+
+    const QString result = Signatures::replace(
+        buffer, QString(), known, Signatures::Position::End);
+
+    QCOMPARE(result, QStringLiteral("Hello.\n"));
+}
+
+void TestSignatures::aBlockMatchingNoKnownSignatureIsNotRemoved()
+{
+    // THE test for the data-loss guard, and it must not be dropped. A "-- "
+    // reaches a buffer without the user ever choosing a signature, pasted in
+    // with quoted text from another client. Replacing from there would delete
+    // everything after it silently.
+    const QStringList known = { QStringLiteral("Jane Doe") };
+    const QString buffer = QStringLiteral(
+        "Hello.\n"
+        "\n"
+        "-- \n"
+        "text the user pasted and wants to keep");
+
+    const QString result = Signatures::replace(
+        buffer, QStringLiteral("Jane Doe"), known, Signatures::Position::End);
+
+    // The user's text survives, and the signature is ADDED. A wrong guess
+    // produces a visible duplicate, never a deletion.
+    QVERIFY(result.contains(
+        QStringLiteral("text the user pasted and wants to keep")));
+    QVERIFY(result.endsWith(QStringLiteral("-- \nJane Doe")));
+}
+
+void TestSignatures::aDelimiterInsideTheQuoteIsNotTheSignature()
+{
+    // The quoted original carries the sender's own signature, quoted. A tail
+    // rule would find it, and under End it would append after it; the block
+    // must not be treated as this message's signature whichever way it goes.
+    //
+    // This test DOCUMENTS the case rather than pinning it, and that is worth
+    // knowing before trying to strengthen it. Two mutations were measured
+    // against it and both stayed green: trimming the delimiter comparison so
+    // that "> -- " matches, and making the quoted text one of the known
+    // signatures so the match guard could not be what refuses the removal.
+    // Neither changes the output, because blockEnd() stops the block at the
+    // quote, so the quoted signature survives whether or not the delimiter
+    // inside it is recognised. The behaviour is correct under both, and no
+    // assertion on the result can separate them.
+    const QStringList known = { QStringLiteral("Jane Doe") };
+    const QString buffer = QStringLiteral(
+        "My reply.\n"
+        "\n"
+        "On Mon, someone wrote:\n"
+        "> the original\n"
+        "> -- \n"
+        "> Their Name\n");
+
+    const QString result = Signatures::replace(
+        buffer, QStringLiteral("Jane Doe"), known, Signatures::Position::End);
+
+    QVERIFY(result.contains(QStringLiteral("> -- \n> Their Name")));
+    QVERIFY(result.endsWith(QStringLiteral("-- \nJane Doe")));
+}
+
+void TestSignatures::aSignatureReadBackFromDiskIsReplaced()
+{
+    // known here is what knownSignatures() produces: text() verbatim, carrying
+    // the trailing newline every editor writes into a file. The block scan
+    // treats a trailing blank line as separation rather than text, so a naive
+    // match compares "Jane Doe" against "Jane Doe\n" and silently fails, and
+    // switching then APPENDS a second signature instead of replacing the first.
+    const QStringList known = { QStringLiteral("Jane Doe\n") };
+    const QString buffer = QStringLiteral("Hello.\n\n-- \nJane Doe\n");
+
+    const QString result = Signatures::replace(
+        buffer, QStringLiteral("Brief"), known, Signatures::Position::End);
+
+    QCOMPARE(result, QStringLiteral("Hello.\n\n-- \nBrief"));
+}
+
+QTEST_MAIN(TestSignatures)
+#include "test_signatures.moc"