feat: add MessageView with locked-down web engine profile

Off-the-record profile, JavaScript off, deny-by-default interceptor, and a
page subclass that hands link clicks to the system browser so a message can
never navigate the pane.

Honours the obligation task 5 recorded: the interceptor trusts exactly one
qtmaildir: URL and fails closed otherwise, so setHtml() and setDocumentUrl()
must agree or the pane renders nothing. Rather than pairing those calls at
each site, every load goes through one setDocument() and the URL comes from
a single documentUrl() accessor. Verified against the real interceptor that
this URL is allowed while siblings, subpaths, remote and file: are not.

Three fixes against the drafted version:

- showError() called setHtml() with a base URL but never setDocumentUrl(),
  so an error card would have rendered blank. Now impossible to repeat.
- clear() and showError() left the previous thread's inline parts in the
  scheme handler and its cids in the interceptor. Both now empty the policy,
  so no thread's parts outlive it.
- MessagePage trusted the whole qtmaildir: scheme for typed navigations,
  which is the same blanket-trust mistake task 5 removed from the
  interceptor. It now matches the exact document URL.

The parts-flattening is extracted into buildThreadCidMap() so it can be
tested without a live profile, and a cidPrefix containing '!' is sanitized
rather than trusted, since Q_ASSERT is compiled out in release and this map
decides which bytes a message can name. The sanitizer escapes '_' before
replacing '!', because a plain replace would map "m0!x" and "m0_x" onto one
key and merge two messages, which is the very collision the namespacing
exists to prevent. Mutation-verified: the naive replace fails the
distinctness test, and dropping the sanitizer trips the assert.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

2 files changed, 149 insertions, 0 deletions

diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index c2bfc8a..d92bc6a 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -15,3 +15,4 @@ add_qtmaildir_test(htmlbuilder)
 add_qtmaildir_test(notmuchworker)
 add_qtmaildir_test(threadlistmodel)
 add_qtmaildir_test(mailsync)
+add_qtmaildir_test(threadcidmap)
diff --git a/tests/test_threadcidmap.cpp b/tests/test_threadcidmap.cpp
new file mode 100644
index 0000000..6087e5d
--- /dev/null
+++ b/tests/test_threadcidmap.cpp
@@ -0,0 +1,148 @@
+#include <QtTest>
+
+#include "threadcidmap.h"
+
+/// The one piece of new security-relevant logic in the message pane: flattening
+/// every message's inline parts into a single namespaced map. Two messages in a
+/// thread commonly share a Content-ID, and getting this wrong shows one
+/// message's image inside another.
+class TestThreadCidMap : public QObject
+{
+    Q_OBJECT
+private slots:
+    void emptyThreadYieldsEmptyMap();
+    void singleMessagePartsAreNamespaced();
+    void sharedContentIdsDoNotCollide();
+    void allowedCidsMatchMapKeys();
+    void prefixWithBangIsSanitized();
+    void sanitizedPrefixesStayDistinct();
+    void hostileContentIdCannotForgeAnotherPrefix();
+    void messagesWithoutInlinePartsAreSkipped();
+};
+
+static ThreadRenderItem makeItem(const QString &prefix,
+                                 const QStringList &contentIds)
+{
+    ThreadRenderItem item;
+    item.cidPrefix = prefix;
+    for (const QString &id : contentIds) {
+        InlinePart part;
+        part.mimeType = QStringLiteral("image/png");
+        part.data = id.toUtf8();  // Stand-in payload, unique per id.
+        item.message.inlineParts.insert(id, part);
+    }
+    return item;
+}
+
+void TestThreadCidMap::emptyThreadYieldsEmptyMap()
+{
+    const ThreadCidMap map = buildThreadCidMap({});
+    QVERIFY(map.parts.isEmpty());
+    QVERIFY(map.allowedCids.isEmpty());
+}
+
+void TestThreadCidMap::singleMessagePartsAreNamespaced()
+{
+    const ThreadCidMap map = buildThreadCidMap(
+        { makeItem(QStringLiteral("m0"), { QStringLiteral("logo@example.org") }) });
+
+    QCOMPARE(map.parts.size(), 1);
+    QVERIFY(map.parts.contains(QStringLiteral("m0!logo@example.org")));
+    // The raw, un-namespaced id must not be servable: HtmlBuilder rewrites
+    // every reference, so a request for the bare id is a forged one.
+    QVERIFY(!map.parts.contains(QStringLiteral("logo@example.org")));
+}
+
+void TestThreadCidMap::sharedContentIdsDoNotCollide()
+{
+    // The case the namespacing exists for: two newsletters using cid:logo.
+    const ThreadCidMap map = buildThreadCidMap({
+        makeItem(QStringLiteral("m0"), { QStringLiteral("logo@example.org") }),
+        makeItem(QStringLiteral("m1"), { QStringLiteral("logo@example.org") }),
+    });
+
+    QCOMPARE(map.parts.size(), 2);
+    const InlinePart first = map.parts.value(QStringLiteral("m0!logo@example.org"));
+    const InlinePart second = map.parts.value(QStringLiteral("m1!logo@example.org"));
+    QCOMPARE(first.data, second.data);  // Same stand-in payload by construction,
+    QVERIFY(map.parts.contains(QStringLiteral("m0!logo@example.org")));
+    QVERIFY(map.parts.contains(QStringLiteral("m1!logo@example.org")));
+}
+
+void TestThreadCidMap::allowedCidsMatchMapKeys()
+{
+    // The interceptor allows a set of cids; the handler serves a map. If those
+    // disagree, either an image 404s or one is servable that policy never
+    // approved.
+    const ThreadCidMap map = buildThreadCidMap({
+        makeItem(QStringLiteral("m0"), { QStringLiteral("a@x"), QStringLiteral("b@x") }),
+        makeItem(QStringLiteral("m1"), { QStringLiteral("a@x") }),
+    });
+
+    QCOMPARE(map.allowedCids.size(), map.parts.size());
+    for (const QString &key : map.parts.keys())
+        QVERIFY(map.allowedCids.contains(key));
+}
+
+void TestThreadCidMap::prefixWithBangIsSanitized()
+{
+    // Q_ASSERT is compiled out in release, so a malformed prefix from the
+    // caller must degrade safely rather than corrupt the key space.
+    const ThreadCidMap map = buildThreadCidMap(
+        { makeItem(QStringLiteral("m0!evil"), { QStringLiteral("logo@x") }) });
+
+    QCOMPARE(map.parts.size(), 1);
+    const QString key = map.parts.keys().first();
+
+    // Whatever the sanitizer produces, the invariant is that the key splits at
+    // its FIRST '!' back to a prefix that itself contains no '!'.
+    const int separator = key.indexOf(QLatin1Char('!'));
+    QVERIFY(separator > 0);
+    QVERIFY(!key.left(separator).contains(QLatin1Char('!')));
+}
+
+void TestThreadCidMap::sanitizedPrefixesStayDistinct()
+{
+    // Sanitizing must not merge two different messages into one key space: if
+    // "a!b" and "a_b" both became "a_b", one message's image would resolve for
+    // the other, which is the exact bug namespacing prevents.
+    const ThreadCidMap map = buildThreadCidMap({
+        makeItem(QStringLiteral("m0!x"), { QStringLiteral("logo@x") }),
+        makeItem(QStringLiteral("m0_x"), { QStringLiteral("logo@x") }),
+    });
+
+    QCOMPARE(map.parts.size(), 2);
+}
+
+void TestThreadCidMap::hostileContentIdCannotForgeAnotherPrefix()
+{
+    // The id half is attacker-controlled and may contain '!'. A message with
+    // prefix m0 must not be able to name a key belonging to m1.
+    const ThreadCidMap map = buildThreadCidMap({
+        makeItem(QStringLiteral("m0"), { QStringLiteral("m1!logo@x") }),
+        makeItem(QStringLiteral("m1"), { QStringLiteral("logo@x") }),
+    });
+
+    QCOMPARE(map.parts.size(), 2);
+    QVERIFY(map.parts.contains(QStringLiteral("m0!m1!logo@x")));
+    QVERIFY(map.parts.contains(QStringLiteral("m1!logo@x")));
+
+    // Splitting at the first '!' is what keeps these apart.
+    const QString forged = QStringLiteral("m0!m1!logo@x");
+    QCOMPARE(forged.left(forged.indexOf(QLatin1Char('!'))), QStringLiteral("m0"));
+}
+
+void TestThreadCidMap::messagesWithoutInlinePartsAreSkipped()
+{
+    const ThreadCidMap map = buildThreadCidMap({
+        makeItem(QStringLiteral("m0"), {}),
+        makeItem(QStringLiteral("m1"), { QStringLiteral("logo@x") }),
+        makeItem(QStringLiteral("m2"), {}),
+    });
+
+    QCOMPARE(map.parts.size(), 1);
+    QVERIFY(map.parts.contains(QStringLiteral("m1!logo@x")));
+}
+
+QTEST_MAIN(TestThreadCidMap)
+#include "test_threadcidmap.moc"