Hi;
The Client-side Javascript / typing settings is inherited from the Eclipse Web Tool Platform plugin and is supposed to only work when editing .js files .
Unfortunately the OPL editors currently do not provide (exposed or private) settings to disable the default behaviour of closing braces and parenthesis.
You may want to open the OPL files in a different editor, such as the basic TextEditor, but you will lose all other OPL specific functionalities

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Frederic Delhoume
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