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.h b/src/cardlayout.h
new file mode 100644
index 0000000..d06ed92
--- /dev/null
+++ b/src/cardlayout.h
@@ -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.
+ */
+
+#pragma once
+
+#include <QFont>
+#include <QRect>
+#include <QVector>
+
+/// Where everything on a card goes, with no painting and no widget.
+///
+/// Split out from CardDelegate on purpose. A delegate needs a live QPainter and
+/// an exposed view before it draws anything, which is what makes delegate tests
+/// fragile: CLAUDE.md records that viewport()->render() returns a blank image in
+/// several ordinary situations, and that a probe reporting "no ink anywhere" is
+/// far more likely broken than the code it is testing. Every geometric claim
+/// about a card is therefore made here, where a test is a function call.
+///
+/// The card is three lines, always:
+///
+///     sender ................................ date      <- senderRect/dateRect
+///     * subject                    @   v 3 replies      <- subjectRect/expanderRect
+///     [tag] [tag]                                       <- tagRect
+struct CardLayout
+{
+    /// What the model says about the row. Deliberately plain data: the layout
+    /// must be computable in a test without a model or a view.
+    struct Input
+    {
+        bool isMessage = false;
+        int depth = 0;       ///< 0 for a thread root, 1 for a direct reply.
+        int replyCount = 0;  ///< 0 means no expander.
+    };
+
+    /// Width of the account accent bar down a thread card's left edge.
+    ///
+    /// A starting value, not a settled one. Five accounts is enough that two
+    /// colours distinct as chips can read alike as thin stripes, and that can
+    /// only be judged against real cards on the user's own screen and theme
+    /// (Task 10). Widen it there if the accounts are not tellable apart.
+    static constexpr int kAccentWidth = 3;
+
+    /// Horizontal breathing room at the card's edges, measured from the accent
+    /// bar rather than from the card, so text does not sit on the colour.
+    static constexpr int kPaddingX = 8;
+
+    /// Vertical breathing room above the first line and below the last.
+    static constexpr int kPaddingY = 4;
+
+    /// How far one level of reply nesting indents.
+    static constexpr int kIndentStep = 18;
+
+    /// The depth past which nothing indents further.
+    ///
+    /// A mailing-list chain can nest a dozen deep, and without a cap the
+    /// sender is eventually pushed off the right edge. Item 20 accepted that
+    /// deep chains must be capped in the VIEW rather than flattened in the
+    /// model, and this is that cap. Rows past it draw at this depth's indent
+    /// with no marker saying so.
+    static constexpr int kMaxDepth = 4;
+
+    QRect senderRect;
+    QRect dateRect;
+    QRect subjectRect;
+    QRect tagRect;
+
+    /// The reply count's rect, and the click target that toggles the thread.
+    /// Empty when the row has no replies.
+    QRect expanderRect;
+
+    /// The account accent bar down the card's left edge.
+    ///
+    /// Thread cards only. A reply's account is its thread's, stated once at the
+    /// head of the conversation, and a second vertical line in a reply's gutter
+    /// would sit a few pixels from the spine and compete with it. The spine
+    /// carries the accent instead, so an expansion is bounded by one colour
+    /// without ever drawing two lines. Empty on a reply.
+    QRect accentRect;
+
+    /// One full-height vertical line per depth level, outermost first.
+    QVector<QRect> spines;
+
+    /// Where the card's text starts, after any indent.
+    int contentLeft = 0;
+
+    int totalHeight = 0;
+
+    /// The height EVERY row gets, thread and reply alike.
+    ///
+    /// Uniform by design: it keeps setUniformRowHeights(true), which is the
+    /// single cheapest property of this layout, since no scrolling or
+    /// hit-testing arithmetic has to account for rows of differing size. The
+    /// cost is a blank third line on a card with no tags, which was accepted
+    /// explicitly.
+    static int heightFor(const QFont &font);
+
+    /// The font the tag chips and the reply count are drawn in: a size down
+    /// from the card's own, so they read as annotation rather than as a third
+    /// column of content.
+    static QFont smallFont(const QFont &cardFont);
+
+    static CardLayout compute(const Input &input, const QRect &rect,
+                              const QFont &font);
+};