The bug was caused by the "Labels" tool. The fix eliminates that tool, but only in German and only in single window mode.
The user reporting this bug ran into another bug which had been reported earlier. In single window mode, selecting the magnifying glass makes it impossible to switch to other "mouse modes" like text selection and pdf selection. This problem occurs in English and a few other localizations, but there is an easy work around. Hold down the Option key while selecting other tools and then selection works.
A new chapter titled "Working Around Bugs" has been added to the TeXShop manual listing this action. Sometimes it is easier to work around a bug than to fix it, and these cases will be listed in the new chapter. Let us hope that it remains short.