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diff --git a/src/formattoolbar.cpp b/src/formattoolbar.cpp
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+/*
+ * 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 "formattoolbar.h"
+
+#include <QStringList>
+
+
+namespace {
+
+/// Normalises the selection a widget reports into an ordered, in-range pair
+/// that does not split a character.
+///
+/// Three hazards, handled once rather than per function. A backwards drag
+/// reports the anchor AFTER the cursor; a selection can outlive the edit that
+/// shortened the buffer under it; and a boundary can land in the middle of a
+/// surrogate pair, where inserting a token splits one character into two
+/// halves and the result is not valid UTF-16 at all.
+///
+/// The surrogate case is not reachable with an arrow key or the mouse, which
+/// move in whole clusters, but QTextCursor::setPosition accepts such a
+/// position, so any caller computing one arithmetically can produce it: a
+/// draft restore, a find/replace, a template insertion. A boundary sitting on
+/// a LOW surrogate is inside a pair, and moving it back by one puts it before
+/// the whole character.
+///
+/// A COLLAPSED cursor moves back, not outward: nudging the two ends in
+/// opposite directions would turn an empty selection into a two-unit one and
+/// wrap a character the user never selected. A real selection widens, so that
+/// touching any part of a character covers the whole of it.
+void normalise(const QString &text, int &from, int &to)
+{
+    from = qBound(0, from, int(text.size()));
+    to = qBound(0, to, int(text.size()));
+    if (from > to)
+        qSwap(from, to);
+
+    const auto insidePair = [&text](int at) {
+        return at < text.size() && text.at(at).isLowSurrogate();
+    };
+
+    if (from == to) {
+        if (insidePair(from)) {
+            --from;
+            to = from;
+        }
+        return;
+    }
+
+    if (insidePair(from))
+        --from;
+    if (insidePair(to))
+        ++to;
+}
+
+}  // namespace
+
+MarkdownFormat::Edit MarkdownFormat::wrap(const QString &text, int start,
+                                          int end, const QString &token)
+{
+    Edit edit;
+    int from = start;
+    int to = end;
+    normalise(text, from, to);
+
+    edit.text = text;
+    // The closing token first: inserting at `from` would shift `to`.
+    edit.text.insert(to, token);
+    edit.text.insert(from, token);
+
+    if (from == to) {
+        // No selection: the cursor goes BETWEEN the two tokens so typing
+        // continues inside them. Landing after the closing token instead is
+        // the mistake a user notices on the first keystroke.
+        edit.selectionStart = from + token.size();
+        edit.selectionEnd = edit.selectionStart;
+    } else {
+        // The selection is preserved so a second press applies a second token
+        // to the same words without reselecting: bold then italic.
+        edit.selectionStart = from + token.size();
+        edit.selectionEnd = to + token.size();
+    }
+
+    return edit;
+}
+
+MarkdownFormat::Edit MarkdownFormat::link(const QString &text, int start, int end)
+{
+    Edit edit;
+    int from = start;
+    int to = end;
+    normalise(text, from, to);
+
+    const QString label = text.mid(from, to - from);
+
+    edit.text = text;
+    edit.text.replace(from, to - from, QStringLiteral("[%1]()").arg(label));
+
+    if (label.isEmpty()) {
+        // Nothing selected: the label is what gets typed first, so the cursor
+        // goes inside the brackets, one past the '['.
+        edit.selectionStart = from + 1;
+    } else {
+        // The label is written; the URL is what remains, so the cursor goes
+        // inside the parentheses: past '[', the label, ']' and '('.
+        edit.selectionStart = from + label.size() + 3;
+    }
+    edit.selectionEnd = edit.selectionStart;
+
+    return edit;
+}
+
+MarkdownFormat::Edit MarkdownFormat::quote(const QString &text, int start, int end)
+{
+    Edit edit;
+    int from = start;
+    int to = end;
+    normalise(text, from, to);
+
+    // Line-based, not a wrap. The selection is widened to whole lines first:
+    // quoting half a line produces markdown that means something else.
+    //
+    // The backwards search starts at `from - 1`, not at `from`. QString's
+    // lastIndexOf INCLUDES the position it is given, so a cursor sitting at
+    // the end of a line, immediately before its newline, would find that
+    // newline and quote the FOLLOWING line instead of the one the cursor is
+    // on. The guard against a negative position matters too, since -1 means
+    // "search from the end" and would find the last newline in the buffer.
+    const int firstLineStart =
+        from > 0 ? text.lastIndexOf(QLatin1Char('\n'), from - 1) + 1 : 0;
+
+    // No newline after the last line, so the end of the text is the end of
+    // the block. Without this the whole tail would be dropped.
+    int lastLineEnd = text.indexOf(QLatin1Char('\n'), to);
+    if (lastLineEnd < 0)
+        lastLineEnd = text.size();
+
+    const QString before = text.left(firstLineStart);
+    const QString middle = text.mid(firstLineStart, lastLineEnd - firstLineStart);
+    const QString after = text.mid(lastLineEnd);
+
+    const QStringList lines = middle.split(QLatin1Char('\n'));
+
+    // Nesting rather than toggling, per the spec: a second press deepens the
+    // quote. There is deliberately no live toggle here, because tracking "my
+    // text" and "the quote" as separate pieces to make one reversible is
+    // machinery for a case the user answers by closing the composer.
+    QStringList result;
+    result.reserve(lines.size());
+    for (const QString &line : lines) {
+        // A blank line keeps the marker, since that is what continues a quote
+        // block in markdown, but WITHOUT the trailing space: several editors
+        // and mail clients strip trailing whitespace, and stripping it from
+        // "> " leaves ">" anyway, so writing it bare is the same result
+        // reached deliberately.
+        result.append(line.isEmpty() ? QStringLiteral(">")
+                                     : QStringLiteral("> ") + line);
+    }
+
+    const QString replacement = result.join(QLatin1Char('\n'));
+    edit.text = before + replacement + after;
+    // The quoted block stays selected, so a second press nests it.
+    edit.selectionStart = firstLineStart;
+    edit.selectionEnd = firstLineStart + replacement.size();
+
+    return edit;
+}