Compare this which is like old squeak selectionAsStream SmalltalkEditor>>selectionForDoitAsStream ^ ReadWriteStream on: self string from: self startIndex to: self stopIndex - 1 and this which has been changed conforming to Cuis TextEditor>>selectionAsStream "Answer a ReadStream on the text in the paragraph that is currently selected." ^ReadStream on: (self string copyFrom: self startIndex to: self stopIndex - 1) (or self selection asString readStream) The two differ. The former stream position starts at editor startIndex - 1, while the later stream position starts at 0. The stream position is used by the Compiler/Parser for notifying the SmalltalkEditor (or TextMorph) requestor. Now look at SmalltalkEditor>>notify: aString at: anInteger in: aStream "The compilation of text failed. The syntax error is noted as the argument, aString. Insert it in the text at starting character position anInteger." | pos | pos := self selectionInterval notEmpty ifTrue: [self startIndex + anInteger - 1 ] ifFalse: [anInteger]. self insertAndSelect: aString at: (pos max: 1) You see that it adds (startIndex - 1) to the error message insertion position which is a Cuis change. (The old ParagraphEditor did not). When #notify:at:in: is used for evaluating #selectionAsStream, all is OK, the (startIndex - 1) is added only once. But when it is used with #selectionForDoitAsStream, the offset is counted twice (because already in the stream location passed in anInteger parameter). I guess the hack #selectionForDoitAsStream that undo the Cuis clean-up has been added because some other part of Parser/Encoder/Compiler is messing with the requestor selection (like highlighting some part of the source for user interaction in case of ambiguity). Tthis is not a valid workaround. In Squeak, I did revert the Cuis clean-up just to keep some simple compatibility with ParagraphEditor, but Pharo does not care about that, so it should apply full simplification. I suggest removing #selectionForDoitAsStream, and replace senders with #selectionAsStream. Then if there are some other feature broken we'll fix'em. OK? Nicolas