Consider this code: w := SystemWindow labelled: 'LatestInspector'. t := TabGroupMorph new addPage: (RectangleMorph new color: Color green ) label: 'Instance' ; addPage: (RectangleMorph new color: Color red) label: 'Methods' . w openInWorld w addMorph: t fullFrame: (LayoutFrame fractions: (0@0 corner: 1@1) offsets: (0@30 corner: 0@0)) w openInWorld . The tab will not have the labels in the right place. This only happens if (1) you actually set colors to the RectangleMorphs (2) The TabGroupMorph is embedded in another morph (SystemWindow (3) There are at least two tabs (for 1 tab, it works great). I suspect the code triggered by color: to be guilty. It triggers a change to the morph, which then triggers relabelling, presumably. I filed this as bug http://code.google.com/p/pharo/issues/detail?id=1316.