Since the crash reported didn't let me report the bug, I copied the kcrash file here:

Application: kdiff3 (1.9.70 (64 bit))

Qt Version: 5.15.8
Frameworks Version: 5.102.0
Operating System: Linux 5.15.0-58-generic x86_64
Windowing System: Wayland
Distribution: KDE neon 5.26
DrKonqi: 5.26.5 [KCrashBackend]

-- Information about the crash:
KDiff3 is using default configuration, and is started using "git mergetool"
After fixing the merge conflict, I want to delete some duplicate lines. Selecting the lines works (using shift+down keys), but on pressing the delete-key KDiff3 crashes.

The crash can be reproduced every time. (tried three times)

-- Backtrace:
Application: KDiff3 (kdiff3), signal: Aborted

[KCrash Handler]
#4  __pthread_kill_implementation (no_tid=0, signo=6, threadid=140625170659776) at ./nptl/pthread_kill.c:44
#5  __pthread_kill_internal (signo=6, threadid=140625170659776) at ./nptl/pthread_kill.c:78
#6  __GI___pthread_kill (threadid=140625170659776, signo=signo@entry=6) at ./nptl/pthread_kill.c:89
#7  0x00007fe5dd572476 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#8  0x00007fe5dd5587f3 in __GI_abort () at ./stdlib/abort.c:79
#9  0x00007fe5dd5b96f6 in __libc_message (action=action@entry=do_abort, fmt=fmt@entry=0x7fe5dd70bb8c "%s\n") at ../sysdeps/posix/libc_fatal.c:155
#10 0x00007fe5dd5d0d7c in malloc_printerr (str=str@entry=0x7fe5dd70ed40 "malloc(): unaligned tcache chunk detected") at ./malloc/malloc.c:5664
#11 0x00007fe5dd5d545c in tcache_get (tc_idx=<optimized out>) at ./malloc/malloc.c:3195
#12 __GI___libc_malloc (bytes=40) at ./malloc/malloc.c:3313
#13 0x00007fe5dd90d9cc in operator new(unsigned long) () from /lib/x86_64-linux-gnu/libstdc++.so.6
#14 0x00007fe5de248d4f in ?? () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#15 0x00007fe5de2452e0 in QTextFormatCollection::indexForFormat(QTextFormat const&) () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#16 0x00007fe5de233bd6 in QTextEngine::indexFormats() () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#17 0x00007fe5de233de0 in QTextEngine::setFormats(QVector<QTextLayout::FormatRange> const&) () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#18 0x00007fe5de2350a3 in QTextLayout::setFormats(QVector<QTextLayout::FormatRange> const&) () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#19 0x0000562d631c4838 in ?? ()
#20 0x0000562d631c62d9 in ?? ()
#21 0x00007fe5de882808 in QWidget::event(QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#22 0x00007fe5de83e793 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#23 0x00007fe5de84650e in QApplication::notify(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#24 0x00007fe5ddd4507a in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#25 0x00007fe5de89e4ff in ?? () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#26 0x00007fe5de83e793 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#27 0x00007fe5ddd4507a in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#28 0x00007fe5de12dbea in QGuiApplicationPrivate::processKeyEvent(QWindowSystemInterfacePrivate::KeyEvent*) () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#29 0x00007fe5de105a5c in QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#30 0x00007fe5dbf01664 in ?? () from /lib/x86_64-linux-gnu/libQt5WaylandClient.so.5
#31 0x00007fe5dc326d3b in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#32 0x00007fe5dc37b6c8 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#33 0x00007fe5dc3243e3 in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#34 0x00007fe5ddd9ead8 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#35 0x00007fe5ddd4399b in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#36 0x00007fe5ddd4bf34 in QCoreApplication::exec() () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#37 0x0000562d631437c4 in ?? ()
#38 0x00007fe5dd559d90 in __libc_start_call_main (main=main@entry=0x562d63142600, argc=argc@entry=13, argv=argv@entry=0x7ffee0e04348) at ../sysdeps/nptl/libc_start_call_main.h:58
#39 0x00007fe5dd559e40 in __libc_start_main_impl (main=0x562d63142600, argc=13, argv=0x7ffee0e04348, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffee0e04338) at ../csu/libc-start.c:392
#40 0x0000562d63147bd5 in ?? ()
[Inferior 1 (process 19987) detached]

Reported using DrKonqi