SUMMARY

Several times today, I've observed an infinite loop in KWin::Placement::placeSmart() when opening a new application.

That leads to kwin_wayland becoming unresponsive until killed, in a 100% CPU loop. I think this is the same root issue as bug #407221, but since that was closed I thought better to file a new bug.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Fedora 38 with KDE 5.27.8

When it gets stuck, `KWin::Placement::placeSmart` is spinning in the overall do/while loop:
`do { ... } while ((overlap != none) && (overlap != h_wrong) && (y < area.bottom()));`

When it reaches the condition at the end of the loop, `y=1234` and area = `{xp = 0, yp = 0, w = 2194.2857142857142, h = 1234.2857142857142}`

With the mismatch between integer y and fractional screen height, I think the root cause is fractional scaling (I have a small 4k display and 175% scaling.

Backtrace:
(gdb) bt
#0  KWin::Placement::placeSmart(KWin::Window*, QRectF const&, KWin::PlacementPolicy) (this=<optimized out>, area=..., window=<optimized out>) at /usr/src/debug/kwin-5.27.8-1.fc38.x86_64/src/placement.cpp:201
#1  KWin::Placement::placeSmart(KWin::Window*, QRectF const&, KWin::PlacementPolicy) (this=<optimized out>, window=<optimized out>, area=...) at /usr/src/debug/kwin-5.27.8-1.fc38.x86_64/src/placement.cpp:159
#2  0x00007f10dd566e0e in KWin::X11Window::manage(unsigned int, bool) (this=0x5633e2e12eb0, w=<optimized out>, isMapped=false) at /usr/src/debug/kwin-5.27.8-1.fc38.x86_64/src/x11window.cpp:757
#3  0x00007f10dd546068 in KWin::Workspace::createX11Window(unsigned int, bool) (this=0x5633e154d140, windowId=12582920, is_mapped=false) at /usr/src/debug/kwin-5.27.8-1.fc38.x86_64/src/workspace.cpp:711
#4  0x00007f10dd44c455 in KWin::Workspace::workspaceEvent(xcb_generic_event_t*) (this=0x5633e154d140, e=0x5633e1bc0120) at /usr/src/debug/kwin-5.27.8-1.fc38.x86_64/src/events.cpp:215
#5  0x00007f10dbab123c in QAbstractEventDispatcher::filterNativeEvent(QByteArray const&, void*, long*) () at /lib64/libQt5Core.so.5
#6  0x00005633e0a4c0d9 in KWin::Xwl::Xwayland::dispatchEvents(KWin::Xwl::Xwayland::DispatchEventsMode) ()
#7  0x00007f10dbae8481 in void doActivate<false>(QObject*, int, void**) () at /lib64/libQt5Core.so.5
#8  0x00007f10dbaeae8d in QSocketNotifier::activated(QSocketDescriptor, QSocketNotifier::Type, QSocketNotifier::QPrivateSignal) () at /lib64/libQt5Core.so.5
#9  0x00007f10dbaeb6fb in QSocketNotifier::event(QEvent*) () at /lib64/libQt5Core.so.5
#10 0x00007f10db1aeb75 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /lib64/libQt5Widgets.so.5
#11 0x00007f10dbab41a8 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /lib64/libQt5Core.so.5
#12 0x00007f10dbb037ff in QEventDispatcherUNIXPrivate::activateSocketNotifiers() () at /lib64/libQt5Core.so.5
#13 0x00007f10dbb03b80 in QEventDispatcherUNIX::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /lib64/libQt5Core.so.5
#14 0x00005633e0b199c2 in QUnixEventDispatcherQPA::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
#15 0x00007f10dbab2b6b in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /lib64/libQt5Core.so.5
#16 0x00007f10dbabadfb in QCoreApplication::exec() () at /lib64/libQt5Core.so.5
#17 0x00005633e0a32e44 in main ()