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diff --git a/tests/test_markdownrenderer.cpp b/tests/test_markdownrenderer.cpp
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+++ b/tests/test_markdownrenderer.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 "markdownrenderer.h"
+
+/// The extension configuration cmark-gfm renders the composer's body with.
+///
+/// No QApplication is needed here: MarkdownRenderer is a pure function over
+/// strings, so QTEST_APPLESS_MAIN avoids pulling in a platform plugin for a
+/// test that has nothing to do with widgets.
+class TestMarkdownRenderer : public QObject
+{
+    Q_OBJECT
+private slots:
+    void commonMarkBasicsRender();
+    void autolinkTurnsABareUrlIntoALink();
+    void strikethroughRenders();
+    void tasklistRenders();
+    void tablesAreNotEnabled();
+    void rawHtmlIsSuppressed();
+    void accentedTextSurvivesAsUtf8();
+    void emptyInputProducesEmptyOutput();
+};
+
+void TestMarkdownRenderer::commonMarkBasicsRender()
+{
+    const QString html = MarkdownRenderer::toHtml(
+        QStringLiteral("**bold** *italic* `code`"));
+    QVERIFY2(html.contains(QStringLiteral("<strong>")), qPrintable(html));
+    QVERIFY2(html.contains(QStringLiteral("<em>")), qPrintable(html));
+    QVERIFY2(html.contains(QStringLiteral("<code>")), qPrintable(html));
+}
+
+void TestMarkdownRenderer::autolinkTurnsABareUrlIntoALink()
+{
+    // The whole reason cmark-gfm was chosen over plain cmark. Under
+    // CommonMark a bare URL is text, and a bare URL in mail is expected to
+    // be clickable.
+    const QString html = MarkdownRenderer::toHtml(
+        QStringLiteral("see https://example.org for details"));
+    QVERIFY2(html.contains(QStringLiteral("<a href=\"https://example.org\"")),
+             qPrintable(html));
+}
+
+void TestMarkdownRenderer::strikethroughRenders()
+{
+    const QString html = MarkdownRenderer::toHtml(QStringLiteral("~~gone~~"));
+    QVERIFY2(html.contains(QStringLiteral("<del>gone</del>")), qPrintable(html));
+}
+
+void TestMarkdownRenderer::tasklistRenders()
+{
+    // Known ceiling: many mail clients strip the checkbox, so those
+    // recipients see the item with no marker. The plain part still carries
+    // the literal "- [ ]", so nothing is lost, only the HTML rendering.
+    const QString html = MarkdownRenderer::toHtml(
+        QStringLiteral("- [ ] todo\n- [x] done"));
+    QVERIFY2(html.contains(QStringLiteral("type=\"checkbox\"")), qPrintable(html));
+    QVERIFY2(html.contains(QStringLiteral("checked")), qPrintable(html));
+}
+
+void TestMarkdownRenderer::tablesAreNotEnabled()
+{
+    // Deliberately off: tables render badly across mail clients regardless of
+    // who generates them. The extension EXISTS in the library, so this
+    // asserts a decision rather than a limitation, and would silently start
+    // passing the wrong way if someone attached it "for completeness".
+    const QString html = MarkdownRenderer::toHtml(
+        QStringLiteral("| a | b |\n|---|---|\n| 1 | 2 |"));
+    QVERIFY2(!html.contains(QStringLiteral("<table")), qPrintable(html));
+    QVERIFY2(html.contains(QStringLiteral("| a | b |")), qPrintable(html));
+}
+
+void TestMarkdownRenderer::rawHtmlIsSuppressed()
+{
+    // CMARK_OPT_SAFE. The body is the user's own text, but a body that can
+    // inject markup into its own generated HTML part is a sharp edge with no
+    // upside.
+    const QString html = MarkdownRenderer::toHtml(
+        QStringLiteral("<script>alert(1)</script>\n\nafter"));
+    QVERIFY2(!html.contains(QStringLiteral("<script>")), qPrintable(html));
+    QVERIFY2(html.contains(QStringLiteral("after")), qPrintable(html));
+}
+
+void TestMarkdownRenderer::accentedTextSurvivesAsUtf8()
+{
+    // This user writes Italian, so accented text is every message rather
+    // than an edge case, and a UTF-8 round trip through a C library is
+    // exactly where it would be lost.
+    const QString source = QString::fromUtf8("perch\xC3\xA9 \xC3\xA8 cos\xC3\xAC");
+    const QString html = MarkdownRenderer::toHtml(source);
+    QVERIFY2(html.contains(source), qPrintable(html));
+}
+
+void TestMarkdownRenderer::emptyInputProducesEmptyOutput()
+{
+    // reply_no_quote opens a composer with an empty body and it must not
+    // produce a stray paragraph or crash the renderer.
+    const QString html = MarkdownRenderer::toHtml(QString());
+    QVERIFY2(html.trimmed().isEmpty(), qPrintable(html));
+}
+
+QTEST_APPLESS_MAIN(TestMarkdownRenderer)
+#include "test_markdownrenderer.moc"