Created attachment 104752 [details] enlarged line showing the problem When I stretch a selected part of a line, it shows an ugly blur. My father uses KolourPaint to draw electrical components and it's kinda problematic to have that modification in the lines because it makes you loose time fixing it in order to get uniformity. I've tested this on Linux Mint 18.1 Mate and Cinnamon (Kolourpaint v4.14.16), and in Fedora 25 KDE, XFCE (Kolourpaint v16.12.2)
Does keeping the "Shift" key pressed while creating the lines helps to resolve the issue?
Without looking into the code, I assume what you want is not feasible. kolourpaint is a pxel-oriented graphics program, not a line drawing application. When you finished constructing a line, it's no longer a line but just a set of pixels. kolourpaint does scale an image or a selection of it via a smooth-scale algorithm which I believe is the right thing to do in a pixel oriented graphics program, and this leads to what you see as blur. I believe there are better solutions for handling line based drawings, where each line stays a line.