SUMMARY Editing strokes made with the calligraphy tool causes all the points to fly off the screen if you move up or left beyond the bounding box of the stroke. STEPS TO REPRODUCE 1. Make a calligraphy stroke 2. Drag any point on the stroke up or left with the edit paths tool 3. drag until you exit the bounding box of all the points on the stroke OBSERVED RESULT All the rest of the points fly wildly in the opposite direction to where you're dragging. EXPECTED RESULT Only the point you're editing should move. SOFTWARE/OS VERSIONS Both windows and Linux appimage NOTES: This was once reported and resolved (Bug 335659) in 2014 as a duplicate of Bug 313913 from 2013, despite that bug being for an unmaintained piece of software called Karbon. All i can gather from this is that Krita somehow was built on top of Karbon and that its bugs are inherited, but I have my doubts that the bug report is visible due to being in the wrong software category, so I'm making a new report in 2019 for Krita 4.2.6.
I can confirm this for the 4.3.0 prealpha (git 307e910) appimage.
Karbon was a sister application of Krita in the Calligra project. When we took Krita out of Calligra, we inherited a lot of shared code. The calligraphy tool actually is one of the plugins that were available in all calligra applications. Since we've split up, it's correct to have this bug reported for Krita, and yes, Karbon is sadly unmaintained. Another option would be to reopen https://bugs.kde.org/show_bug.cgi?id=335659, but let's not go there :-) Thanks for making this report!
This bug should be fixed after this merge: https://invent.kde.org/kde/krita/merge_requests/194