This happens occasionally. KMail consumes 100% of one CPU core for many hours, then eventually stops. Restart doesn't help.

When attaching gdb and interrupting it, almost always one gets a trace similar to this:

(gdb) bt
#0  0x00007fc5bca098b3 in QTextEngine::validate() const () from /usr/lib/libQt5Gui.so.5
#1  0x00007fc5bca10792 in QTextEngine::itemize() const () from /usr/lib/libQt5Gui.so.5
#2  0x00007fc5bca1321d in QTextEngine::findItem(int, int) const () from /usr/lib/libQt5Gui.so.5
#3  0x00007fc5bca14e04 in QTextEngine::shapeLine(QScriptLine const&) () from /usr/lib/libQt5Gui.so.5
#4  0x00007fc5bca21c62 in QTextLine::cursorToX(int*, QTextLine::Edge) const () from /usr/lib/libQt5Gui.so.5
#5  0x00007fc5bca6aa64 in QTextCursorPrivate::setX() () from /usr/lib/libQt5Gui.so.5
#6  0x00007fc5bca6f658 in QTextCursor::insertText(QString const&, QTextCharFormat const&) () from /usr/lib/libQt5Gui.so.5
#7  0x00007fc5bca700ba in QTextCursor::insertText(QString const&) () from /usr/lib/libQt5Gui.so.5
#8  0x00007fc5ba2131ba in Sonnet::Highlighter::slotRehighlight() () from /usr/lib/libKF5SonnetUi.so.5
#9  0x00007fc5bc5b3a7c in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib/libQt5Core.so.5
#10 0x00007fc5bc5bfbe8 in QTimer::timeout(QTimer::QPrivateSignal) () from /usr/lib/libQt5Core.so.5
#11 0x00007fc5bc5b430b in QObject::event(QEvent*) () from /usr/lib/libQt5Core.so.5
#12 0x00007fc5bcf01e14 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/libQt5Widgets.so.5
#13 0x00007fc5bcf096e1 in QApplication::notify(QObject*, QEvent*) () from /usr/lib/libQt5Widgets.so.5
#14 0x00007fc5bc589c39 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /usr/lib/libQt5Core.so.5
#15 0x00007fc5bc5dc39a in QTimerInfoList::activateTimers() () from /usr/lib/libQt5Core.so.5
#16 0x00007fc5bc5dcc5a in ?? () from /usr/lib/libQt5Core.so.5
#17 0x00007fc5b89d93cf in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#18 0x00007fc5b89daf89 in ?? () from /usr/lib/libglib-2.0.so.0
#19 0x00007fc5b89dafce in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#20 0x00007fc5bc5dcfc9 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQt5Core.so.5
#21 0x00007fc5a0463e12 in ?? () from /usr/lib/libQt5XcbQpa.so.5
#22 0x00007fc5bc5888cc in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQt5Core.so.5
#23 0x00007fc5bc590bc6 in QCoreApplication::exec() () from /usr/lib/libQt5Core.so.5
#24 0x000055d46759af60 in ?? ()
#25 0x00007fc5bbfe9223 in __libc_start_main () from /usr/lib/libc.so.6
#26 0x000055d46759b11e in _start ()