feat: render tags as chips instead of a text column

Spelled out per row, tags ran to 500 pixels of largely repeated text and
took most of the thread list's width. The column is gone; tags render as
coloured chips split by what they actually mean.

An account tag says which mailbox a thread arrived in, and draws as a
chip in front of the subject. A functional tag says what state a thread
is in, and those fill one row under the message pane. One row keeps the
message area from shifting between threads with different tag counts, so
whatever does not fit collapses into a +N chip that names the rest in its
tooltip.

TagColors resolves a colour by exact tag first, then by top-level prefix,
so a single "shopping" entry covers shopping/amazon and shopping/nike
while shopping/amazon can still override its own. That matters at 96
tags. Built-in defaults cover the usual state tags, and anything left
unconfigured falls back to a hash of the name, stable so a chip does not
change colour as the list scrolls.

1 files changed, 63 insertions, 0 deletions

diff --git a/src/tagstrip.h b/src/tagstrip.h
new file mode 100644
index 0000000..4102bed
--- /dev/null
+++ b/src/tagstrip.h
@@ -0,0 +1,63 @@
+/*
+ * 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 <QStringList>
+#include <QWidget>
+
+class TagColors;
+
+/// One row of tag chips under the message pane.
+///
+/// A single row by design: the message area must not shift as you move between
+/// threads with different numbers of tags. Whatever does not fit collapses
+/// into a trailing "+N" chip whose tooltip names the hidden tags.
+class TagStrip : public QWidget
+{
+    Q_OBJECT
+public:
+    explicit TagStrip(QWidget *parent = nullptr);
+
+    /// Not owned; must outlive the strip.
+    void setTagColors(const TagColors *colours);
+
+    /// Account tags are filtered out: they belong to the thread list chip,
+    /// being a different taxonomy from the functional tags shown here.
+    void setTags(const QStringList &tags);
+
+    QSize sizeHint() const override;
+
+    /// The tags actually drawn, in order. Exposed for testing the overflow
+    /// split without rendering.
+    QStringList visibleTags() const { return m_visible; }
+    QStringList hiddenTags() const { return m_hidden; }
+
+protected:
+    void paintEvent(QPaintEvent *event) override;
+    void resizeEvent(QResizeEvent *event) override;
+
+private:
+    /// Recomputes the visible/hidden split for the current width.
+    void relayout();
+
+    QStringList m_tags;     ///< Functional tags only, account ones removed.
+    QStringList m_visible;
+    QStringList m_hidden;
+    const TagColors *m_tagColors = nullptr;
+};