If you make a calculation that results in a negative number in the hundreds, then a thousands separator (a comma here in the US) is incorrectly displayed at the beginning of the number, right after the minus sign. The same goes for a negative result in the hundred-thousands (and so on I presume). For example, take 100 and subtract 1000, the result is displayed like "-,900". Or take 50 and subtract 1000000, the result is displayed as "-,999,950". My version is 17.04.2, but this wasn't available in the version selection.
https://phabricator.kde.org/D6326
Git commit 50cfdbb7703ab676498125d1fc8380af910a9f3a by Christoph Feck. Committed on 22/06/2017 at 16:20. Pushed by cfeck into branch 'Applications/17.04'. Fix grouping display with negative numbers FIXED-IN: 17.04.3 Differential Revision: https://phabricator.kde.org/D6326 M +10 -1 kcalcdisplay.cpp https://commits.kde.org/kcalc/50cfdbb7703ab676498125d1fc8380af910a9f3a