feat(rules): preview a rule's mail in the thread list

Item 77. The dialog could say how many messages a rule matched and not
which ones. A Preview in list button now runs the selected rule's query
in the main window; the dialog stays open, since comparing the rule
against its results is the point.

Two constraints from the backlog entry, both now asserted and both
mutation-checked.

The query runs exactly as stored, with no tag:new and no wrapping
parentheses. The post-new hook adds those when it applies a rule, and a
preview that copied them would match nothing outside a sync window,
since tag:new is set only on mail that has just arrived.

The account selector is cleared first. runQuery() wraps the bar's text
in the selected account's scope, and a rule query usually names its own
path already, so previewing one with an account selected would scope it
twice and show an empty list, which reads as "this rule collects no
mail".

The second mutation only fails once the test's config has an account to
select: with the default empty config the selector sits on "All
accounts" anyway, and asserting that a preview leaves it there passed
against the mutation. Recorded in the test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 43 insertions, 0 deletions

diff --git a/src/mainwindow.cpp b/src/mainwindow.cpp
index e8b4dda..e2df6de 100644
--- a/src/mainwindow.cpp
+++ b/src/mainwindow.cpp
@@ -1324,6 +1324,9 @@ void MainWindow::showTagRulesDialog()
     // setFolders refills the rows that exist by then.
     QMetaObject::invokeMethod(m_worker, "requestFolders", Qt::QueuedConnection);
 
+    connect(dialog, &TagRulesDialog::previewRequested,
+            this, &MainWindow::onRulePreviewRequested);
+
     connect(dialog, &TagRulesDialog::countsRequested, this, [this, dialog]() {
         QMetaObject::invokeMethod(
             m_worker, "requestMessageCounts", Qt::QueuedConnection,
@@ -1570,6 +1573,46 @@ void MainWindow::onCountsReady(const QVector<int> &counts, quint64 generation)
         m_messageView->showPlaceholder(placeholderHelpers());
 }
 
+QString MainWindow::queryTextForTesting() const
+{
+    return m_queryEdit->text();
+}
+
+QString MainWindow::selectedAccountForTesting() const
+{
+    return m_accountBox->currentData().toString();
+}
+
+void MainWindow::selectAccountForTesting(const QString &key)
+{
+    const int index = m_accountBox->findData(key);
+    if (index >= 0)
+        m_accountBox->setCurrentIndex(index);
+}
+
+void MainWindow::onRulePreviewRequested(const QString &query)
+{
+    // Unscoped, deliberately. runQuery() wraps the bar's text in the selected
+    // account's scope, and a rule query usually names its own path already
+    // (path:"work/**" is what every account rule looks like), so previewing
+    // one with an account selected would scope it twice and match nothing.
+    // That reads as "this rule collects no mail", which is the opposite of
+    // what the preview is for.
+    m_accountBox->setCurrentIndex(0);
+
+    // Through the query bar, like onPlaceholderQueryRequested: the bar then
+    // shows what is on screen and the user can edit the rule's query there
+    // before deciding to change the rule itself.
+    m_queryEdit->setText(query);
+    runCurrentQuery();
+
+    // The dialog is a separate window and may be covering this one or sitting
+    // beside it. Raising makes the result visible either way, and the dialog
+    // stays open so the two can be compared.
+    raise();
+    activateWindow();
+}
+
 void MainWindow::onPlaceholderQueryRequested(const QString &query)
 {
     // Through the query bar rather than straight to the worker, so the bar