@wim I have another observation. The problem has persisted for some days in a particular Layout Detail. I have been noticing it, but ignoring it on the understanding that a fix would resolve it at some stage. The Detail in question is a copy of another Detail on the same Layout page, with minor layer visibility differences. I deleted the Detail and copied it again from the other Detail, then made the appropriate layer visibility changes. Bingo. The hidden lines no longer showed through. It appears that the Detail was remembering the fault.