Each time I connect and disconnect the display, plasmashell increases memory usage by around 50MB and it never frees that. The biggest leak found by profiling with heaptrack is this (sorry for lack of symbols):

QImageData::create(QSize const&, QImage::Format) in ?? (libQt5Gui.so.5)
QImage::QImage(QSize const&, QImage::Format) in ?? (libQt5Gui.so.5)
<unresolved function> in ?? (libQt5Gui.so.5)
<unresolved function> in ?? (libQt5Gui.so.5)
QImageReader::read(QImage*) in ?? (libQt5Gui.so.5)
<unresolved function> in ?? (libQt5Quick.so.5)
<unresolved function> in ?? (libQt5Quick.so.5)
<unresolved function> in ?? (libQt5Quick.so.5)
<unresolved function> in ?? (libQt5Quick.so.5)
QApplicationPrivate::notify_helper(QObject*, QEvent*) in ?? (libQt5Widgets.so.5)
QApplication::notify(QObject*, QEvent*) in ?? (libQt5Widgets.so.5)
QCoreApplication::notifyInternal2(QObject*, QEvent*) in ?? (libQt5Core.so.5)
QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) in ?? (libQt5Core.so.5)
<unresolved function> in ?? (libQt5Core.so.5)
g_main_context_dispatch in ?? (libglib-2.0.so.0)
<unresolved function> in ?? (libglib-2.0.so.0)
g_main_context_iteration in ?? (libglib-2.0.so.0)
QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) in ?? (libQt5Core.so.5)
QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) in ?? (libQt5Core.so.5)
QThread::exec() in ?? (libQt5Core.so.5)
<unresolved function> in ?? (libQt5Quick.so.5)
<unresolved function> in ?? (libQt5Core.so.5)
start_thread in ?? (libpthread.so.0)
__GI___clone in ?? (libc.so.6)


I can easily reproduce it so I can try to do more debugging if you have any ideas. It's not clear to me if it's a problem in plasma or Qt.

Hardware:
- Thinkpad T470p with Gefore 940MX
- Samsung display connected by DisplayPort
Sofware:
- Archlinux
- plasma-workspace 5.12.3
- Qt 5.10.1-6
- mesa 17.3.7
- Xorg 1.19.6+13+gd0d1a694f-1
- glxinfo:
Extended renderer info (GLX_MESA_query_renderer):
    Vendor: Intel Open Source Technology Center (0x8086)
    Device: Mesa DRI Intel(R) HD Graphics 630 (Kaby Lake GT2)  (0x591b)
    Version: 17.3.7
    Accelerated: yes
    Video memory: 3072MB
    Unified memory: yes