Posting here because the bug tracker login is looping for me; happy for a maintainer
to move this into Jira. Minimal 40-line repro + crash stack below.

What happens

On Windows, dragging the second-to-last dock widget out of a floating tabbed
group (QDockWidgetGroupWindow) crashes with an access violation. The group drops
from 2 members to 1, Qt destroys the group window and reparents the surviving dock
to the main window, and QDockAreaLayoutInfo::reparentWidgets() dereferences a
dangling item.widgetItem while QMainWindowLayout::applyState() replays the
drag's saved layout state.

• A 3-member group torn down to 2 does not crash — only the transition that
  destroys the group window (2→1).
• It's not drag-specific: programmatically removing one member of a 2-member
  group via addDockWidget() + setFloating() also crashes, just deferred ~1 s.
  Removing both (2→0, empty group) crashes too — so the fault is the group-window
  teardown itself.

Looks like a Windows regression / uncovered case of
QTBUG-118579 (fixed 6.5.4 / 6.6.2 /
6.7.0 for Linux/X11 + macOS). That fix — the qobject_cast<QDockWidgetGroupWindow*>
skip in reparentWidgets — is present in 6.11.1 but runs after the faulting
dereference.

Environment

Qt 6.11.1, Windows 11, MSVC x64 (reproduced with the stock msvc2022_64 shared
kit and with a from-source static build).

Steps to reproduce

1. Build & run the program below (3 dock widgets open floating).
2. Drag two of them together into one tabbed floating group.
3. Drag one of those two tabs back out and drop it. → crash.

Crash stack

QDockAreaLayoutInfo::reparentWidgets(QWidget*)            qdockarealayout.cpp:2210
QMainWindowLayout::applyState(QMainWindowLayoutState&, bool)  qmainwindowlayout.cpp:3240
QMainWindowLayout::restore(QInternal::SaveStateRule)         qmainwindowlayout.cpp:2727
QDockWidgetPrivate::endDrag(EndDragMode)                     qdockwidget.cpp:833
QDockWidgetPrivate::setFloating(bool)                        qdockwidget.cpp:1485
QDockWidget::setFloating(bool)                              qdockwidget.cpp:1474
QDockWidgetGroupWindow::reparentToMainWindow(QDockWidget*)  qdockwidget.cpp:772
QDockWidgetPrivate::endDrag(EndDragMode)                    qdockwidget.cpp:860
QMainWindowTabBar::mouseReleaseEvent(QMouseEvent*)          qmainwindowlayout.cpp:2111

The AV reads the item.widgetItem value (seen as 0xFFFFFFFFFFFFFFFF and
0x0000000100000001 across runs — a stale/garbage pointer in item_list):

// qdockarealayout.cpp, QDockAreaLayoutInfo::reparentWidgets
if (item.widgetItem) {
    QWidget *w = item.widgetItem->widget();        // <-- AV (item.widgetItem dangling)
    if (qobject_cast<QDockWidgetGroupWindow *>(w)) // QTBUG-118579 skip, runs too late
        continue;
    ...

Minimal reproducer

main.cpp:

#include <QApplication>
#include <QMainWindow>
#include <QDockWidget>
#include <QPlainTextEdit>
#include <QLabel>

int main(int argc, char** argv)
{
    QApplication app(argc, argv);

    QMainWindow w;
    w.resize(1000, 650);
    w.setDockOptions(QMainWindow::AllowTabbedDocks
                   | QMainWindow::AllowNestedDocks
                   | QMainWindow::GroupedDragging);
    w.setCentralWidget(new QLabel("central", &w));

    for (int i = 1; i <= 3; ++i) {
        auto* d = new QDockWidget(QString("Dock %1").arg(i), &w);
        d->setWidget(new QPlainTextEdit(QString("content %1").arg(i)));
        d->setAllowedAreas(Qt::AllDockWidgetAreas);
        w.addDockWidget(Qt::LeftDockWidgetArea, d);
        d->setFloating(true);
        d->move(120 + i * 80, 120 + i * 70);
        d->resize(360, 260);
    }

    w.show();
    return app.exec();
}

CMakeLists.txt:

cmake_minimum_required(VERSION 3.21)
project(qtdockcrash LANGUAGES CXX)
set(CMAKE_CXX_STANDARD 17)
find_package(Qt6 REQUIRED COMPONENTS Widgets)
qt_standard_project_setup()
qt_add_executable(qtdockcrash WIN32 main.cpp)
target_link_libraries(qtdockcrash PRIVATE Qt6::Widgets)