feat(ui): fill the blank message pane with a branded placeholder

An empty right pane said nothing, and multi-select made it a routine
sight. It now carries the wordmark, thread counts that run their query
when clicked, and a sync line that appears only when something needs
attention.

Rendered into the existing web view as a third document shape, so there
is one document path and one set of security rules. The brand palette is
a deliberate exception to deriving colours from the desktop theme, since
a logo is brand rather than chrome; the theme still picks which of the
two sets is used.

Counts refresh when the pane is about to show rather than in the
background: one goes stale the moment a tag is edited, and refreshing one
nobody is looking at is work for nothing. A generation counter discards a
superseded reply, and a late answer cannot repaint over an opened thread.

The helper lines are real links because JavaScript is off in this
profile. The handler is gated on the placeholder actually being
displayed, so the same URL inside a message body is dropped: a stranger's
mail must not drive the thread list, even to run a harmless query.

Three defects found while building, all silent:

- Every CSS percentage was invalid. QString::arg does not collapse "%%"
  into "%", so the document carried "50%%" and the browser dropped each
  declaration holding one, disabling the mask, the glow and both radial
  gradients while still rendering something plausible. Substitution is by
  named token now, which cannot collide with a percent sign.
- A geometry probe endorsed the layout while that was live, because it
  measured only properties without percentages.
- The font test passed against a build with one face missing, since the
  other satisfied both of its checks on its own.

The mockup's light values needed correcting against a real pane: the grid
vanished at a 2% luminance step on white, and the glow subtracts light
there rather than adding it, washing the pane. Strength only, not hue.

1 files changed, 171 insertions, 0 deletions

diff --git a/tests/test_htmlbuilder.cpp b/tests/test_htmlbuilder.cpp
index 7fa45a2..fdb57c5 100644
--- a/tests/test_htmlbuilder.cpp
+++ b/tests/test_htmlbuilder.cpp
@@ -48,6 +48,15 @@ private slots:
     // Theming.
     void aDarkPaletteProducesADarkDocument();
     void everyColourComesFromThePalette();
+
+    // The placeholder pane (item 30).
+    void placeholderPicksTheBrandSetFromTheDesktopTheme();
+    void placeholderHelperBecomesALinkOnItsQuery();
+    void placeholderHelperWithoutAQueryIsNotALink();
+    void placeholderEscapesHelperText();
+    void placeholderEmbedsItsFontsRatherThanFetchingThem();
+    void placeholderReferencesNoRemoteResource();
+    void placeholderStyleHasNoUnsubstitutedTokens();
     void theBodyAlwaysGetsABackground();
     void aSendersOwnHtmlIsNotRecoloured();
 };
@@ -357,6 +366,168 @@ void TestHtmlBuilder::everyColourComesFromThePalette()
     }
 }
 
+void TestHtmlBuilder::placeholderPicksTheBrandSetFromTheDesktopTheme()
+{
+    // The brand colours are fixed, deliberately: this is the one place where
+    // the desktop palette does NOT supply the values. What the desktop decides
+    // is which of the two sets is used, and getting that backwards is the
+    // failure the item warns about, a light lockup on a dark desktop.
+    const HtmlBuilder::BrandPalette dark = HtmlBuilder::brandPaletteFrom(
+        makeTestPalette(QColor(0x1a, 0x1a, 0x1a), QColor(0xee, 0xee, 0xee)));
+    const HtmlBuilder::BrandPalette light = HtmlBuilder::brandPaletteFrom(
+        makeTestPalette(QColor(0xff, 0xff, 0xff), QColor(0x11, 0x11, 0x11)));
+
+    QCOMPARE(dark.background.name(), QStringLiteral("#060b10"));
+    QCOMPARE(light.background.name(), QStringLiteral("#ffffff"));
+
+    // Not merely different: the right way round. A set whose background is
+    // darker than its text is the dark set, whichever values it holds.
+    QVERIFY(dark.background.lightnessF() < dark.title.lightnessF());
+    QVERIFY(light.background.lightnessF() > light.title.lightnessF());
+}
+
+void TestHtmlBuilder::placeholderHelperBecomesALinkOnItsQuery()
+{
+    // JavaScript is off in this profile, so a helper can only be actionable by
+    // being a real link that the page's navigation handler intercepts.
+    const QString html = HtmlBuilder::buildPlaceholder(
+        { { QStringLiteral("12 unread"), QStringLiteral("tag:unread") } },
+        QStringLiteral("0.10.0"),
+        HtmlBuilder::brandPaletteFrom(QPalette()));
+
+    QVERIFY(html.contains(QStringLiteral("href=\"qtmaildir-query:tag%3Aunread\"")));
+    QVERIFY(html.contains(QStringLiteral("12 unread")));
+}
+
+void TestHtmlBuilder::placeholderHelperWithoutAQueryIsNotALink()
+{
+    // The sync line reports a state rather than naming a search, so clicking it
+    // must not run an empty query and wipe the thread list.
+    const QString html = HtmlBuilder::buildPlaceholder(
+        { { QStringLiteral("3 edits waiting to sync"), QString() } },
+        QStringLiteral("0.10.0"),
+        HtmlBuilder::brandPaletteFrom(QPalette()));
+
+    QVERIFY(html.contains(QStringLiteral("3 edits waiting to sync")));
+    QVERIFY(!html.contains(QStringLiteral("qtmaildir-query:")));
+}
+
+void TestHtmlBuilder::placeholderEscapesHelperText()
+{
+    // A helper label carries a count this code produced, but the query half is
+    // built from configuration and a saved query is user-written. Neither may
+    // reach the document unescaped, and the query is doubly encoded: percent
+    // for the URL, then HTML for the attribute.
+    const QString html = HtmlBuilder::buildPlaceholder(
+        { { QStringLiteral("<script>alert(1)</script>"),
+            QStringLiteral("tag:\"a\"><script>") } },
+        QStringLiteral("0.10.0"),
+        HtmlBuilder::brandPaletteFrom(QPalette()));
+
+    QVERIFY(!html.contains(QStringLiteral("<script>")));
+    QVERIFY(html.contains(QStringLiteral("&lt;script&gt;")));
+}
+
+void TestHtmlBuilder::placeholderEmbedsItsFontsRatherThanFetchingThem()
+{
+    // The mockup @imports Google Fonts, which the interceptor blocks by design.
+    // The fonts ship in the binary and must arrive as data: URIs, or the pane
+    // silently falls back to a system font and stops looking like the brand.
+    const QString html = HtmlBuilder::buildPlaceholder(
+        {}, QStringLiteral("0.10.0"),
+        HtmlBuilder::brandPaletteFrom(QPalette()));
+
+    QVERIFY(!html.contains(QStringLiteral("fonts.googleapis.com")));
+
+    // BOTH faces, each with a real payload. Counting @font-face rules or
+    // checking the document's total size passes with one face missing, since
+    // the other is large enough on its own to carry either check: a mutation
+    // pointing one src at a nonexistent resource survived exactly that test.
+    // A missing resource yields an empty src, so the length is what catches it.
+    static const QRegularExpression src(
+        QStringLiteral("src: url\\('data:font/woff2;base64,([^']*)'\\)"));
+    auto it = src.globalMatch(html);
+    int faces = 0;
+    while (it.hasNext()) {
+        ++faces;
+        QVERIFY(it.next().captured(1).size() > 1000);
+    }
+    QCOMPARE(faces, 2);
+}
+
+void TestHtmlBuilder::placeholderReferencesNoRemoteResource()
+{
+    // The load-bearing security check, asserted as a negative for the same
+    // reason everyColourComesFromThePalette is: one leftover reference is the
+    // entire defect, and it would be invisible because the interceptor blocks
+    // it and the pane just renders slightly wrong.
+    const QString html = HtmlBuilder::buildPlaceholder(
+        { { QStringLiteral("12 unread"), QStringLiteral("tag:unread") } },
+        QStringLiteral("0.10.0"),
+        HtmlBuilder::brandPaletteFrom(QPalette()));
+
+    QVERIFY(!html.contains(QStringLiteral("//fonts")));
+    QVERIFY(!html.contains(QStringLiteral("@import")));
+
+    // The one http: URL is the SVG namespace, which is an identifier and never
+    // fetched. Asserting its exact value rather than excluding the scheme
+    // wholesale: a second http: URL appearing later would be a real resource.
+    static const QRegularExpression http(QStringLiteral("http://[^\"' ]*"));
+    auto plain = http.globalMatch(html);
+    while (plain.hasNext()) {
+        QCOMPARE(plain.next().captured(0),
+                 QStringLiteral("http://www.w3.org/2000/svg"));
+    }
+
+    // Every https: URL must be the footer's website link, which is a link the
+    // user clicks and not a resource the document fetches.
+    static const QRegularExpression https(QStringLiteral("https://[^\"' ]*"));
+    auto it = https.globalMatch(html);
+    while (it.hasNext()) {
+        QCOMPARE(it.next().captured(0),
+                 QStringLiteral("https://danix.xyz/qtmaildir"));
+    }
+}
+
+void TestHtmlBuilder::placeholderStyleHasNoUnsubstitutedTokens()
+{
+    // The defect this exists for shipped once and was invisible. The template
+    // used QString::arg with "%%" for every CSS percentage, and arg() does NOT
+    // collapse "%%" into "%", so the stylesheet reached the browser carrying
+    // "50%%". Each declaration holding one was dropped as invalid, which
+    // silently disabled the grid mask, the glow and both radial gradients. The
+    // pane still rendered, still looked plausible, and a geometry probe that
+    // happened to measure only percentage-free properties reported it correct.
+    const QString html = HtmlBuilder::buildPlaceholder(
+        { { QStringLiteral("12 unread"), QStringLiteral("tag:unread") } },
+        QStringLiteral("0.10.0"),
+        HtmlBuilder::brandPaletteFrom(QPalette()));
+
+    const qsizetype start = html.indexOf(QStringLiteral("<style>"));
+    const qsizetype end = html.indexOf(QStringLiteral("</style>"));
+    QVERIFY(start >= 0 && end > start);
+    const QString style = html.mid(start, end - start);
+
+    // No doubled percent survives into the document.
+    QVERIFY2(!style.contains(QStringLiteral("%%")),
+             "the stylesheet carries '%%', which CSS rejects: every rule "
+             "containing one is silently dropped");
+
+    // No token went unreplaced. A renamed colour would otherwise leave
+    // '@ACCENT@' sitting in the CSS as a dropped declaration.
+    static const QRegularExpression token(QStringLiteral("@[A-Z_]+@"));
+    const QRegularExpressionMatch leftover = token.match(style);
+    QVERIFY2(!leftover.hasMatch(),
+             qPrintable(QStringLiteral("unsubstituted token '%1' in the "
+                                       "stylesheet").arg(leftover.captured(0))));
+
+    // The three effects the bug disabled, each asserted by name so that
+    // deleting one is a test failure rather than a silent visual regression.
+    QVERIFY(style.contains(QStringLiteral("mask-image")));
+    QVERIFY(style.contains(QStringLiteral("radial-gradient")));
+    QVERIFY(style.contains(QStringLiteral("aspect-ratio")));
+}
+
 void TestHtmlBuilder::theBodyAlwaysGetsABackground()
 {
     // The original CSS set no background at all, which is why the pane was