fix: render the message pane at all

Clicking a thread left the pane blank. Two independent bugs, both from the
same false premise: that setHtml() navigates to the base URL it is given.

It does not. setHtml() navigates to a data: URL carrying the markup and
applies the base URL afterwards, purely as the document's origin. Verified
empirically on Qt 6.11.

Built on that wrong assumption were:

- MessagePage::acceptNavigationRequest compared the navigation's URL
  against documentUrl() and rejected everything else, so the document load
  was refused. It now accepts a typed main-frame navigation, which is one
  we initiated ourselves.
- RequestInterceptor exempted exactly the qtmaildir: base URL and denied
  everything else, so the data: document load was blocked too.

The interceptor fix is scoped to ResourceTypeMainFrame rather than allowing
the data: scheme outright. A blanket allow would have been a real hole: a
message body can write <img src="data:..."> or an iframe, and the existing
dataSchemeBlocked test in test_interceptor.cpp was right to fail when that
was tried. Sub-resource data: URLs remain denied.

Note this was never working. The drafted version had the same defect in a
different spelling (it compared url.scheme() rather than the whole URL, and
would have rejected the data: navigation just the same), and task 11 shipped
with no runtime test to catch it. test_messageview.cpp now pins all three
facts: the document loads, its text reaches the page, and a data: image
inside a hostile body stays blocked.

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

2 files changed, 36 insertions, 6 deletions

diff --git a/src/messageview.cpp b/src/messageview.cpp
index 4e7586e..f8dc4ab 100644
--- a/src/messageview.cpp
+++ b/src/messageview.cpp
@@ -31,10 +31,18 @@ protected:
     bool acceptNavigationRequest(const QUrl &url, NavigationType type,
                                  bool isMainFrame) override
     {
-        // setHtml() arrives as a typed navigation to our own base URL. Matching
-        // the exact URL rather than the scheme keeps this consistent with the
-        // interceptor, which deliberately refuses to trust qtmaildir: wholesale.
-        if (type == NavigationTypeTyped && url == MessageView::documentUrl())
+        // setHtml() does NOT navigate to the base URL it is given: it
+        // navigates to a data: URL carrying the markup, and applies the base
+        // URL afterwards as the document's origin. Verified empirically on Qt
+        // 6.11; an earlier version of this function compared against
+        // documentUrl() here and rejected every document load, so nothing
+        // rendered at all.
+        //
+        // A typed main-frame navigation is therefore one we initiated
+        // ourselves, and is accepted on that basis. This is not the security
+        // boundary: RequestInterceptor still vets every request the document
+        // goes on to make, including the qtmaildir: origin itself.
+        if (type == NavigationTypeTyped && isMainFrame)
             return true;
 
         if (type == NavigationTypeLinkClicked) {
diff --git a/src/requestinterceptor.cpp b/src/requestinterceptor.cpp
index 8fd2419..6267b64 100644
--- a/src/requestinterceptor.cpp
+++ b/src/requestinterceptor.cpp
@@ -14,8 +14,13 @@ bool RequestInterceptor::shouldAllow(const QUrl &url)
     // by unusual casing, in either the allow or the deny direction.
     const QString scheme = url.scheme();
 
-    // The document itself is loaded via setHtml() with a qtmaildir: base URL,
-    // so a request for exactly that URL must pass or nothing renders at all.
+    // data: is never allowed here. It is permitted for the main-frame
+    // document only, which is handled in interceptRequest() where the resource
+    // type is known: a message body can put data: in <img src> or
+    // <iframe src>, and those must stay blocked.
+
+    // The document's origin is the qtmaildir: base URL, so requests can still
+    // arrive on that scheme once the document is live.
     // This is the ONLY trusted qtmaildir: URL: everything else on this scheme
     // is denied, including sub-paths of it. A hostile message body can put
     // arbitrary qtmaildir: URLs in <img src>, <link href>, etc., so this
@@ -64,6 +69,23 @@ bool RequestInterceptor::shouldAllow(const QUrl &url)
 
 void RequestInterceptor::interceptRequest(QWebEngineUrlRequestInfo &info)
 {
+    // The main-frame document arrives as a data: URL, because setHtml() does
+    // not fetch the base URL it is given: it navigates to a data: URL carrying
+    // the markup and applies the base URL afterwards as the document's origin.
+    // (Verified empirically on Qt 6.11. The qtmaildir: rule in shouldAllow()
+    // was written on the opposite assumption, and until this was found every
+    // document load was blocked and the pane rendered blank.)
+    //
+    // Scoping this to ResourceTypeMainFrame is what keeps it from being a
+    // hole: those bytes are the ones HtmlBuilder produced a moment earlier and
+    // they arrive in the navigation itself rather than over any transport,
+    // while a data: URL written into a message body reaches this function as
+    // an image, stylesheet or subframe and is still denied by shouldAllow().
+    if (info.resourceType() == QWebEngineUrlRequestInfo::ResourceTypeMainFrame
+        && info.requestUrl().scheme() == QLatin1String("data")) {
+        return;
+    }
+
     if (!shouldAllow(info.requestUrl()))
         info.block(true);
 }