feat(view): compute a card's geometry with no painting

Split from the delegate deliberately. A delegate needs a live painter and an
exposed view, which is what makes delegate tests fragile: viewport()->render()
returns a blank image in several ordinary situations, and a probe reporting no
ink is likelier broken than the code it tests. Every geometric claim about a
card is made here, where a test is a function call.

Three lines at a uniform height, so setUniformRowHeights(true) survives. Indent
caps at depth 4 with qMin rather than a branch, so depth 5 and depth 50 land in
the same place. The date is measured before the sender, so a long sender elides
instead of painting over it.

Two traps handled that a first pass gets wrong. QRect::right() is inclusive, so
the right edge is carried as an exclusive one and everything sized from it lands
where the padding constant says rather than a pixel short. And QFont::pointSizeF
returns -1 for a font set in pixels, which qt6ct does, so smallFont branches on
which unit the font actually carries instead of silently returning the card's
own size.

1 files changed, 119 insertions, 0 deletions

diff --git a/src/cardlayout.cpp b/src/cardlayout.cpp
new file mode 100644
index 0000000..8d952b2
--- /dev/null
+++ b/src/cardlayout.cpp
@@ -0,0 +1,119 @@
+/*
+ * 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 "cardlayout.h"
+
+#include <QFontMetrics>
+
+QFont CardLayout::smallFont(const QFont &cardFont)
+{
+    QFont small = cardFont;
+    // Derived from the card's font rather than fixed, so it follows the
+    // desktop's font size instead of shrinking to nothing on a large one.
+    //
+    // pointSizeF() returns -1 for a font set in PIXELS, which qt6ct and some
+    // styles do. Subtracting from -1 would ask for an invalid size and Qt
+    // would silently keep the original, making the small font the same size as
+    // the card's; the pixel branch avoids that.
+    if (small.pointSizeF() > 0.0)
+        small.setPointSizeF(qMax(6.0, cardFont.pointSizeF() - 1.0));
+    else if (small.pixelSize() > 0)
+        small.setPixelSize(qMax(8, cardFont.pixelSize() - 1));
+    return small;
+}
+
+int CardLayout::heightFor(const QFont &font)
+{
+    const QFontMetrics metrics(font);
+    const QFontMetrics smallMetrics(smallFont(font));
+    // Two lines at the card's font, one at the small one, plus the padding
+    // above the first and below the last.
+    return kPaddingY * 2 + metrics.height() * 2 + smallMetrics.height();
+}
+
+CardLayout CardLayout::compute(const Input &input, const QRect &rect,
+                               const QFont &font)
+{
+    CardLayout out;
+    const QFontMetrics metrics(font);
+    const QFontMetrics smallMetrics(smallFont(font));
+
+    out.totalHeight = rect.height();
+
+    // The accent bar sits flush against the card's left edge, on thread cards
+    // only, and everything else starts after it so no text sits on the colour.
+    if (!input.isMessage) {
+        out.accentRect =
+            QRect(rect.left(), rect.top(), kAccentWidth, rect.height());
+    }
+    const int textLeft = rect.left() + kAccentWidth;
+
+    // Indent, capped. qMin rather than a branch so depth 5 and depth 50 land
+    // in exactly the same place.
+    const int depth = qMin(input.depth, kMaxDepth);
+    const int indent = depth * kIndentStep;
+    out.contentLeft = textLeft + kPaddingX + indent;
+
+    // One spine per level actually indented, each running the card's full
+    // height so an expansion reads as one continuous block.
+    for (int level = 0; level < depth; ++level) {
+        const int x = textLeft + kPaddingX + level * kIndentStep
+                      + kIndentStep / 2;
+        out.spines.append(QRect(x, rect.top(), 2, rect.height()));
+    }
+
+    // The EXCLUSIVE right edge: one past the last pixel a card may draw on.
+    // QRect::right() is inclusive (left + width - 1), so building widths from
+    // it directly lands everything one pixel short of the intended padding.
+    const int right = rect.right() + 1 - kPaddingX;
+    const int lineOneTop = rect.top() + kPaddingY;
+    const int lineTwoTop = lineOneTop + metrics.height();
+    const int lineThreeTop = lineTwoTop + metrics.height();
+
+    // The date is measured first and the sender gets what is left, so a long
+    // sender is elided rather than painting over the date.
+    const int dateWidth = metrics.horizontalAdvance(
+        QStringLiteral("8888-88-88 88:88"));
+    out.dateRect = QRect(right - dateWidth, lineOneTop, dateWidth,
+                         metrics.height());
+    out.senderRect = QRect(out.contentLeft, lineOneTop,
+                           qMax(0, out.dateRect.left() - out.contentLeft
+                                       - kPaddingX),
+                           metrics.height());
+
+    // The expander is the reply count, on line two and on the right.
+    if (input.replyCount > 0) {
+        const int countWidth = smallMetrics.horizontalAdvance(
+            QStringLiteral("▾ 8888 replies"));
+        out.expanderRect = QRect(right - countWidth, lineTwoTop, countWidth,
+                                 metrics.height());
+    }
+
+    const int subjectRight = out.expanderRect.isEmpty()
+                                 ? right
+                                 : out.expanderRect.left() - kPaddingX;
+    out.subjectRect = QRect(out.contentLeft, lineTwoTop,
+                            qMax(0, subjectRight - out.contentLeft),
+                            metrics.height());
+
+    out.tagRect = QRect(out.contentLeft, lineThreeTop,
+                        qMax(0, right - out.contentLeft),
+                        smallMetrics.height());
+
+    return out;
+}