Sometimes, when using instant preview, the final result shown on the canvas can have some sort of tile glitch, that goes away after switching off/on the visibility of the layer. I was doing this kind of testing when noticing this bug: -On a new document, A4 300ppi size, with instant preview activated -Fill the canvas with some colors -Select Smudge_textured brush preset, set it to 1000px, and make lot of strokes moving colors on all the canvas, with strokes overlapping each others. -Go take a coffee or two while rendering the strokes at real size -Check the result Reproducible: Sometimes
*** This bug has been marked as a duplicate of bug 353754 ***