Bug when dragging a layer/object around with gridlines turned on and set to Percent. Details:
(1) Open a PNG file in Pixelmator (mine is a screenshot 1024x791, 1.1 Mb in size.)
(2) Copy & paste a small portion of the image (say 64x64 pixels) which create a new layer for it.
(3) Turn on the grid and in the preferences set Gridline every 10 Percent (or any other number). Make sure the 2 alignment options are checked on.
(4) Using the move tool on the layer with the smaller image, drag the image around and notice how the guides appear in non-sensical locations rather than on the gridlines or centres of document.
(5) Go back to preferences and change Gridlines every to Inches or any of the other units.
(6) Follow step 4 again but this time notice how the smaller image now snaps to the gridlines appropriately.
Mid-2010 Mac Pro
macOS Sierra 10.12.1 (16B2555)
AMD Radeon HD 7950 3GB
PixelmatorVersion 3.6 Cordillera (61103)