Verify desktop image previews decode asynchronously and paint without file I/O

tools/chatgui/tests/test_desktop_commands.cpp

@@ -6,6 +6,7 @@
 #include "../db/db_manager.h"
 #include "../src/inputbar.h"
 #include "../src/messagedelegate.h"
+#include "../src/messagelist.h"
 #include "../src/sound_manager.h"
 #include <QtTest>
 #include <QTemporaryDir>
@@ -186,6 +187,33 @@ private slots:
         QCOMPARE(outstanding.size(), size_t(0));
 #endif
     }
+    void imagePreviewPreparedOutsidePaint() {
+        QTemporaryDir directory; QVERIFY(directory.isValid());
+        const QString path = directory.filePath("image.png");
+        QImage original(1200, 1800, QImage::Format_ARGB32); original.fill(Qt::red); QVERIFY(original.save(path));
+        MessageList history; history.resize(600, 500); history.show();
+        auto* model = history.findChild<QStandardItemModel*>(); QVERIFY(model);
+        auto* view = history.findChild<QListView*>(); QVERIFY(view);
+        auto* item = new QStandardItem;
+        item->setData("image/png", MsgContentTypeRole); item->setData(path, MsgVoiceFileRole);
+        item->setData(1200, MsgImageWidthRole); item->setData(1800, MsgImageHeightRole);
+        item->setData(2, MsgFileDownloadStateRole); model->appendRow(item);
+        QTRY_VERIFY_WITH_TIMEOUT(!item->data(MsgImagePreviewRole).value<QImage>().isNull(), 3000);
+        const QImage preview = item->data(MsgImagePreviewRole).value<QImage>();
+        QVERIFY(preview.width() <= 600 && preview.height() <= 600);
+        QCOMPARE(preview.width() * 3, preview.height() * 2); QVERIFY(!item->data(MsgImageErrorRole).toBool());
+        QVERIFY(QFile::remove(path)); // Paint обязан использовать подготовленное превью даже без файла на диске.
+        QImage canvas(600, 400, QImage::Format_ARGB32); canvas.fill(Qt::black);
+        QStyleOptionViewItem option; option.initFrom(view); option.rect = canvas.rect();
+        QPainter painter(&canvas); view->itemDelegate()->paint(&painter, option, model->index(0, 0)); painter.end();
+        int red = 0;
+        for (int y = 0; y < canvas.height(); ++y) for (int x = 0; x < canvas.width(); ++x) {
+            const QColor color = canvas.pixelColor(x, y);
+            if (color.red() > 220 && color.green() < 30 && color.blue() < 30) ++red;
+        }
+        QVERIFY(red > 1000);
+    }
+
     void boundCommand_data() {
         QTest::addColumn<QString>("scenario");
         for (const auto& name : {"unavailable", "stopped", "cancelled", "executed"}) QTest::newRow(name) << QString(name);