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Have you tried Scrivener? I haven't used it for screenwriting, but it looks like it's got a great tool for it. It's not open source, but $45 is fairly cheap.
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/9492 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/9492 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/