Comment 4 for bug 1606369

After some more testing I came to the conclusion that proportionally balanced shadows (as proposed by David Black) will result in improved appearance when using a light theme (and will be OK for dark themes, too). Also, with that fix in effect, it is sufficient to change the value of org.compiz.profiles.mate.plugins.thumbnail.border from 12 to 15 (instead of 19) in order to completely eliminate the reported artefact. Consequently:

Alternative solution revisited: Apply the fix proposed by David Black AND change the value of org.compiz.profiles.mate.plugins.thumbnail.border from 12 to 15.