fix(ui): expand flat threads, reach the first message, localise the date

Four faults from the first hand test, two of them behavioural.

An expander that opened onto nothing. setThreadMessages kept only nodes with
depth > 0, and notmuch_thread_get_toplevel_messages returns every message at
depth 0 when a thread carries no usable In-Reply-To, so a flat thread
contributed no children while its card still advertised the count. Measured in
the user's database: of 396 inbox threads three are flat, one of them nine
messages long, and every two-message thread of that kind was affected, which is
exactly why the fault looked like "the expander only works with more than one
reply". The rule is now position, not depth: every message except the first,
which is the root card itself. That is also the correct rule rather than a
workaround, since the row under the root is the second message however notmuch
chose to nest it.

The thread's first message was unreachable. Selecting a root card loaded the
whole thread, so the pane showed every message with only the last expanded, and
no row in the list offered the first one: the reply rows are messages two
onward. The root card now renders its own message, which is what the card
already claims to be. It keeps its thread id, unlike the message-row path, so
mark-read and the tag-change repaint still work; that is asserted, because
clearing it is the obvious way to write this and silently disables both. Before
the replies are loaded the model has no first message to name and the whole
thread stays the honest answer.

Dates ignored the locale. One hardcoded "yyyy-MM-dd hh:mm" produced a US-looking
format on an Italian desktop; QLocale::system() now formats it, and the width
reserved for the date comes from the same function so a longer locale cannot
clip.

The expander was a bare number on the card's own background. It is a pill now,
carrying "3 replies" (and "1 reply", singular), sized from the label actually
drawn and measured in both glyph states so it does not resize under the pointer
on click. Its fill is blended from Text toward Base rather than taken from
QPalette::Button, which is #2b2b2b against a Base of #2b2b2b on the user's
theme: byte identical, so the pill was invisible. A theme may make any two roles
equal; a blend is defined against the surface it sits on and cannot collide with
it. Checked by rendering both a dark and a light palette and looking.

1 files changed, 44 insertions, 9 deletions

diff --git a/src/carddelegate.cpp b/src/carddelegate.cpp
index a3a846d..f13205e 100644
--- a/src/carddelegate.cpp
+++ b/src/carddelegate.cpp
@@ -157,7 +157,7 @@ void CardDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option,
     const QDateTime date =
         index.data(ThreadListModel::DateRole).toDateTime();
     painter->drawText(card.dateRect, Qt::AlignVCenter | Qt::AlignRight,
-                      date.toString(QStringLiteral("yyyy-MM-dd hh:mm")));
+                      CardLayout::formatDate(date));
 
     // Line 2: the flag mark, the subject, the attachment mark.
     QString subject = index.data(ThreadListModel::SubjectRole).toString();
@@ -178,16 +178,51 @@ void CardDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option,
                       metrics.elidedText(line2, Qt::ElideRight,
                                          card.subjectRect.width()));
 
-    // The reply count, which is also the expander.
+    // The reply count, which is also the expander, drawn as a PILL.
+    //
+    // A bare "3" on the card's own background read as an unexplained number
+    // beside the subject and gave no hint that it could be clicked. The chip
+    // shape says "this is a control", matching the tag chips on line 3, and the
+    // word says what the number counts.
     if (!card.expanderRect.isEmpty()) {
-        painter->setFont(CardLayout::smallFont(chrome.font));
         const int count = index.data(ThreadListModel::ReplyCountRole).toInt();
-        const QString glyph = (option.state & QStyle::State_Open)
-                                  ? QStringLiteral("▾")
-                                  : QStringLiteral("▸");
-        painter->drawText(card.expanderRect, Qt::AlignVCenter | Qt::AlignRight,
-                          QStringLiteral("%1 %2").arg(glyph).arg(count));
-        painter->setFont(chrome.font);
+        const QString label = CardLayout::expanderLabel(
+            count, option.state & QStyle::State_Open);
+
+        painter->save();
+        painter->setFont(CardLayout::smallFont(chrome.font));
+
+        // Blended from Text toward Base rather than taken from a palette ROLE.
+        // QPalette::Button is the role this obviously wants and it is
+        // #2b2b2b against a Base of #2b2b2b on the user's theme: byte
+        // identical, so the pill was invisible. A theme is free to make any two
+        // roles equal, and several do; a blend cannot collide with the surface
+        // it sits on because it is defined relative to it.
+        //
+        // Toward Text, so it darkens on a light theme and lightens on a dark
+        // one, the same trick replyBackground() and threadLineColour() use.
+        const QColor base = option.palette.color(QPalette::Base);
+        const QColor text = option.palette.color(QPalette::Text);
+        constexpr qreal kFillWeight = 0.18;
+        const QColor fill = QColor::fromRgbF(
+            text.redF() * kFillWeight + base.redF() * (1.0 - kFillWeight),
+            text.greenF() * kFillWeight + base.greenF() * (1.0 - kFillWeight),
+            text.blueF() * kFillWeight + base.blueF() * (1.0 - kFillWeight));
+        painter->setRenderHint(QPainter::Antialiasing, true);
+        painter->setPen(Qt::NoPen);
+        painter->setBrush(fill);
+        // Fully rounded ends, the same shape TagChip paints: the radius is half
+        // the height, so the pill cannot look like a rectangle with soft corners.
+        const qreal radius = card.expanderRect.height() / 2.0;
+        painter->drawRoundedRect(card.expanderRect, radius, radius);
+
+        // The pen is restored from the card's own text colour rather than
+        // ButtonText, which belongs to the role that just proved unreliable.
+        painter->setPen(option.state & QStyle::State_Selected
+                            ? option.palette.highlightedText().color()
+                            : text);
+        painter->drawText(card.expanderRect, Qt::AlignCenter, label);
+        painter->restore();
     }
 
     painter->restore();