refactor(view): stop the view painting, and hit-test the reply count

ThreadListView::paintEvent and its band arithmetic are deleted. The view existed
to paint a strip across five columns; with one column and one delegate painting
the whole card there is nothing to span, and the two faults that arithmetic kept
producing go with it: a deleted row cut in half, and every other row showing a
bare stripe.

What survives is the expander hit-test, because a delegate gets no click of its
own without an editor. It now asks CardDelegate for the rect rather than
recomputing it, so the drawn target and the clickable one cannot drift. The
siblingAtColumn(0) dance is gone: with one column, the index already is column 0.

Item 51 closes here rather than separately. A card is exactly viewport width, so
the view has no horizontal scroll range for a click to scroll into, and the test
asserts that directly.

Two rendering tests had to change how they measure, not merely which index they
name. The indent test asserted on visualRect, which now reports the SAME rect
for a thread and its reply by design, since setIndentation(0) leaves the indent
to CardLayout: it reads contentLeft off the layout instead. And the expander
test reported zero ink over a card the delegate paints 2183 pixels into, because
viewport()->render() returned a blank image, exactly as CLAUDE.md warns; it now
paints the delegate into an image directly and carries a guard proving the probe
can see ink before it reports finding none. Both were mutation-checked.

Two tests are deleted rather than ported. Both existed to prove the row-wide
strip spanned columns a delegate could not reach, which is a property of code
that no longer exists.

1 files changed, 33 insertions, 88 deletions

diff --git a/src/mainwindow.cpp b/src/mainwindow.cpp
index 423aa7b..30148d4 100644
--- a/src/mainwindow.cpp
+++ b/src/mainwindow.cpp
@@ -42,7 +42,7 @@
 #include <QSplitter>
 #include <QStandardPaths>
 #include <QStatusBar>
-#include <QTableView>
+#include <QScrollBar>
 #include <QTimer>
 #include <QToolBar>
 #include <QVBoxLayout>
@@ -52,6 +52,8 @@
 #include "mimeparser.h"
 #include "notmuchworker.h"
 #include "querycompleter.h"
+#include "carddelegate.h"
+#include "cardlayout.h"
 #include "tagchip.h"
 #include "tagdialog.h"
 #include "threadlistmodel.h"
@@ -157,20 +159,9 @@ void MainWindow::restoreUiState()
         m_splitter->restoreState(splitter);
     }
 
-    // A header blob saved against a different set of columns must be
-    // discarded, not restored. QHeaderView::restoreState() returns TRUE for a
-    // blob with fewer sections than the model and applies the old widths to
-    // the wrong columns: adding the attachment column in front shifted every
-    // saved width one place right, silently mangling the layout with no error
-    // to detect it by (verified on Qt 6.11). The column count is stored
-    // alongside and the blob is only used when it still matches.
-    const QByteArray header = state.value(QStringLiteral("threadlist/header"))
-                                  .toByteArray();
-    const int savedColumns =
-        state.value(QStringLiteral("threadlist/columns")).toInt();
-    if (!header.isEmpty() && savedColumns == ThreadListModel::ColumnCount) {
-        m_threadView->header()->restoreState(header);
-    }
+    // No thread-list header state is read. The pane is one column drawn whole
+    // by CardDelegate, so there are no widths to restore; a blob saved by an
+    // older version is simply ignored (item 53's Upgrading note).
 
     // The config value is the starting point for a profile that has never
     // zoomed; once the user does, the state file is what they last had.
@@ -187,11 +178,6 @@ void MainWindow::saveUiState() const
     state.setValue(QStringLiteral("window/geometry"), saveGeometry());
     state.setValue(QStringLiteral("window/state"), saveState());
     state.setValue(QStringLiteral("window/splitter"), m_splitter->saveState());
-    state.setValue(QStringLiteral("threadlist/header"),
-                   m_threadView->header()->saveState());
-    // Guards the blob above: see restoreUiState().
-    state.setValue(QStringLiteral("threadlist/columns"),
-                   int(ThreadListModel::ColumnCount));
     state.setValue(QStringLiteral("message/zoom"), m_messageView->zoomFactor());
 }
 
@@ -543,83 +529,42 @@ void MainWindow::buildUi()
     // delegate is confined to one column's rectangle.
     m_threadView = new ThreadListView(central);
     m_threadView->setModel(m_model);
+    m_threadView->setItemDelegate(new CardDelegate(this));
+    m_threadView->setHeaderHidden(true);
     m_threadView->setSelectionBehavior(QAbstractItemView::SelectRows);
     m_threadView->setSelectionMode(QAbstractItemView::ExtendedSelection);
-    m_threadView->header()->setStretchLastSection(false);
-    // Every column Interactive, Subject included: Stretch and ResizeToContents
-    // both compute a width and discard the user's drag. Nothing absorbs spare
-    // width as a result, so the columns end where they end.
-    for (int column = 0; column < ThreadListModel::ColumnCount; ++column) {
-        m_threadView->header()->setSectionResizeMode(
-            column, QHeaderView::Interactive);
-    }
 
-    // The expander goes on the subject column, not on column 0. Column 0 is the
-    // narrow attachment marker, and an expander there has no room: it pushes the
-    // paperclip out of a 28px column entirely.
-    m_threadView->setTreePosition(ThreadListModel::SubjectColumn);
-
-    // No style-drawn branch decoration. SubjectDelegate draws the expander
-    // itself, because drawBranches runs BEFORE the row's cells and the
-    // delegate's own background paints straight over anything put there: a
-    // 60-pixel triangle survived as 8 pixels, indistinguishable from the
-    // near-invisible dot this replaces. Leaving both enabled would draw the
-    // theme's dot underneath the delegate's glyph.
+    // No style-drawn branch decoration. CardDelegate draws the expander itself,
+    // because drawBranches runs BEFORE the row's cells and the delegate's own
+    // background paints straight over anything put there: a 60-pixel triangle
+    // once survived as 8. Leaving both enabled would draw the theme's dot
+    // underneath the delegate's glyph.
     m_threadView->setRootIsDecorated(false);
 
-    // Wider than Qt's default 20px, and the reason is specific rather than
-    // aesthetic. A thread row carries an account chip in front of its subject
-    // and a reply row does not, so a reply's text already starts roughly a
-    // chip's width (~60px) to the LEFT of its thread's. At the default indent
-    // the 20px shift is swallowed by that difference and the replies read as
-    // flush with the thread, or even outdented. Verified against the running
-    // app, not assumed: visualRect reported a correct 20px indent while the
-    // rendered text showed none, because the geometry is indented and the
-    // delegate then lays the text out from its own left edge.
-    m_threadView->setIndentation(SubjectDelegate::kReplyIndent);
-
-    // Two delegates, and the split is not cosmetic. RowStyleDelegate carries
-    // only the selection fix every column needs: the read/unread dimming
-    // arrives as a Qt::ForegroundRole, which Qt's painting prefers over the
-    // highlight, leaving a selected read row grey on the selection colour.
-    //
-    // SubjectDelegate adds the account chip and the tag pills, and must go on
-    // the subject column ALONE. It reads AccountLabelRole, a property of the
-    // row rather than of a cell, so installed view-wide it draws the chip into
-    // every column: tried once, and the list came out with a chip repeated
-    // four times per row.
-    m_threadView->setItemDelegate(new RowStyleDelegate(this));
-    m_threadView->setItemDelegateForColumn(ThreadListModel::SubjectColumn,
-                                           new SubjectDelegate(this));
+    // Zero, because CardLayout draws the indent itself. Qt's own indentation
+    // would shift the card's rect, and every rect on the card is measured from
+    // that rect's left edge, so the two would compound.
+    m_threadView->setIndentation(0);
 
     // One height for every row. A QTreeView has no vertical header to carry a
-    // default section size, so the height comes from uniformRowHeights plus the
-    // delegate's own sizeHint. uniformRowHeights is not merely an optimisation
-    // here: without it the tree measures every row separately and the tag strip,
-    // which is painted OUTSIDE any cell, is not accounted for in any of those
-    // measurements, so rows collapse to text height and the strip is clipped.
+    // default section size, so the height comes from uniformRowHeights plus
+    // CardDelegate::sizeHint.
     m_threadView->setUniformRowHeights(true);
-    // Widening a column past the viewport scrolls rather than squeezing the
-    // others. Per-pixel so the scroll does not jump a whole column at a time.
-    // Banding, so the eye can follow a row across four columns and a pill
-    // strip without losing it. The colour comes from the palette's
-    // AlternateBase, so it follows the desktop theme.
+
+    // Banding, so the eye can follow a card across the pane. The colour comes
+    // from the palette's AlternateBase, so it follows the desktop theme.
     m_threadView->setAlternatingRowColors(true);
 
-    m_threadView->setHorizontalScrollBarPolicy(Qt::ScrollBarAsNeeded);
-    m_threadView->setHorizontalScrollMode(QAbstractItemView::ScrollPerPixel);
-
-    // Starting widths only; a drag overrides them, and they are what the
-    // saved-widths item will persist.
-    // Without this the attachment column cannot be narrow at all: the default
-    // minimum section size is 58px on this platform, and setColumnWidth()
-    // clamps to it silently rather than reporting the smaller value back.
-    m_threadView->header()->setMinimumSectionSize(24);
-    m_threadView->setColumnWidth(ThreadListModel::AttachmentColumn, 28);
-    m_threadView->setColumnWidth(ThreadListModel::FlagColumn, 28);
-    m_threadView->setColumnWidth(ThreadListModel::DateColumn, 130);
-    m_threadView->setColumnWidth(ThreadListModel::AuthorsColumn, 180);
-    m_threadView->setColumnWidth(ThreadListModel::SubjectColumn, 520);
+    // A card is exactly viewport width, so there is nothing to scroll to
+    // sideways. Turning the bar off is what closes item 51: a click used to
+    // scroll the list horizontally, because the subject column was wider than
+    // the viewport and auto-scroll brought the clicked index fully into view.
+    m_threadView->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
+
+    // Scrolling a whole card at a time rather than a fraction of one, so a
+    // card is never left half above the top edge.
+    m_threadView->verticalScrollBar()->setSingleStep(
+        CardLayout::heightFor(m_threadView->font()));
 
     // Replies are loaded when a thread is expanded, not with the query.
     // Walking the reply tree of every thread in a 10k-thread result would cost
@@ -1061,7 +1006,7 @@ void MainWindow::buildMenus()
     m_threadContextMenu->addAction(m_actions.value(QStringLiteral("select_all")));
 
     m_threadView->setContextMenuPolicy(Qt::CustomContextMenu);
-    connect(m_threadView, &QTableView::customContextMenuRequested,
+    connect(m_threadView, &QWidget::customContextMenuRequested,
             this, &MainWindow::showThreadContextMenu);
 
     // The frequent subset only. A toolbar holding every action is as