From b68ee8a13710894a06f73ec64b638aec80835aab Mon Sep 17 00:00:00 2001
From: Miikka Heikkinen <miikka.heikkinen@qt.io>
Date: Tue, 29 Apr 2025 12:08:21 +0300
Subject: [PATCH] QmlDesigner: Prevent deadlock in drag handling in material
 browser

model->startDrag() starts a new event loop and doesn't exit until the
drag is over, so in the old implementation m_*ToDrag were not
cleared until the drag was complete. However, in some cases it is
possible to receive another move event while drag is still ongoing,
which would then get interpreted as start of another drag as
m_*ToDrag were still valid, causing nested drag event loops, which
caused a deadlock in gui thread.

This issue is prevented by clearing m_*ToDrag before drag starts.

Fixes: QDS-15245
Change-Id: Ib8cd4e5e9f1ab4efcc697e5a6e87d1b2df54ffb7
Reviewed-by: Marco Bubke <marco.bubke@qt.io>
---
 .../materialbrowser/materialbrowserwidget.cpp       | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/plugins/qmldesigner/components/materialbrowser/materialbrowserwidget.cpp b/src/plugins/qmldesigner/components/materialbrowser/materialbrowserwidget.cpp
index e8f297b4e99..6097812e2b8 100644
--- a/src/plugins/qmldesigner/components/materialbrowser/materialbrowserwidget.cpp
+++ b/src/plugins/qmldesigner/components/materialbrowser/materialbrowserwidget.cpp
@@ -115,20 +115,25 @@ bool MaterialBrowserWidget::eventFilter(QObject *obj, QEvent *event)
                 QByteArray internalId;
 
                 if (isMaterial) {
-                    internalId.setNum(m_materialToDrag.internalId());
+                    qint32 internalIdVal = m_materialToDrag.internalId();
+                    internalId.setNum(internalIdVal);
+                    m_materialToDrag = {};
+                    m_textureToDrag = {};
                     mimeData->setData(Constants::MIME_TYPE_MATERIAL, internalId);
                     model->startDrag(std::move(mimeData),
                                      m_previewImageProvider->requestPixmap(
-                                         QString::number(m_materialToDrag.internalId()),
+                                         QString::number(internalIdVal),
                                          nullptr,
                                          {128, 128}),
                                      this);
                 } else {
+                    QString sourcePropVal = m_textureToDrag.variantProperty("source").value().toString();
                     internalId.setNum(m_textureToDrag.internalId());
+                    m_textureToDrag = {};
                     mimeData->setData(Constants::MIME_TYPE_TEXTURE, internalId);
                     QString iconPath = QLatin1String("%1/%2")
                                     .arg(DocumentManager::currentResourcePath().path(),
-                                         m_textureToDrag.variantProperty("source").value().toString());
+                                         sourcePropVal);
 
                     QPixmap pixmap;
                     const QString suffix = iconPath.split('.').last().toLower();
@@ -144,8 +149,6 @@ bool MaterialBrowserWidget::eventFilter(QObject *obj, QEvent *event)
                             ":/propertyeditor/images/texture_default.png");
                     model->startDrag(std::move(mimeData), pixmap.scaled({128, 128}), this);
                 }
-                m_materialToDrag = {};
-                m_textureToDrag = {};
             }
         }
     } else if (event->type() == QMouseEvent::MouseButtonRelease) {