I downgraded to JMRI 3.10.1, and the pull down menus for Circuit Builder in Panel Editor worked okay. They said that a detector circuit was not defined. When I created the first OBlock, the error message ended.
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Then I re-installed v. 4.12 and the pull down menus in Circuit Builder were jumbled again. Apparently the regular choices were superimposed with the error message, a detector circuit is not defined. When I selected the first choice, to add a detector circuit, I could create an OBlock. Then the jumbled characters of the error message stopped, and the other choices displayed clearly.
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There seems to be a bug in v. 4.12 for the Circuit Builder that causes the pull down menus to display improperly before any OBlocks are created. Once one is defined, the problem ends. Perhaps the next release can address this problem.