In the color selector which open for the fill layer or when you clic on the foregroud/background color on the toolbar, you cant clic and drag to quickly select a pure white or black. When you drag, there is some lag and the color stop changing when you are outside the triangle. The advanced color selector widget and the popup color selector works just fine.
This seems to be the same as https://bugs.kde.org/show_bug.cgi?id=372221, link for reference.
Yes this is the same bug but not on the same color selector, for me the on canvas popup color selector and the widget works fine.
*** This bug has been marked as a duplicate of bug 372221 ***