Trelby Screenwriting Program; free and opensource https://www.trelby.org/
The program description does not mention portability, and I have not tested it, so we don't know if this application is portable.
But because Trelby is free and open source and Softpedia has no record for it, I believe it deserves a mention here at TPFC, because in addition TPFC does have a screenwriting app (I may be wrong).
From the Trelby web site:
A free, multiplatform, feature-rich screenwriting (screenplay editor) program!
Trelby is simple, fast and elegantly laid out to make screenwriting simple. It is infinitely configurable.
Trelby is free software, that you can contribute to.
Trelby - Features
Screenplay editor: Enforces correct script format and pagination, auto-completion, and spell checking.
Multiplatform : Behaves identically on all platforms, generating the exact same output.
Choice of view: Multiple views, including draft view, WYSIWYG mode, and full screen to suit your writing style.
Name database: Character name database containing over 200,000 names from various countries.
Reporting: Scene/location/character/dialogue reports.
Compare: Ability to compare scripts, so you know what changed between versions.
Import: Screenplay formatted text, Final Draft XML (.fdx), Celtx (.celtx), Fountain (.fountain), Adobe Story (.astx) and Fade In Pro (.fadein).
Export: PDF, formatted text, HTML, RTF, Final Draft XML (.fdx) and Fountain (.fountain).
PDF: Built-in, highly configurable PDF generator. Supports embedding your chosen font. Also supports generating PDFs with custom watermarks, to help track shared files.
Free software: Licensed under the GPL, Trelby welcomes developers and screenwriters to contribute in making it more useful.
Home page: https://www.trelby.org/
Screenshots: https://www.trelby.org/screenshot/
Download: https://www.trelby.org/download/
Manual: https://www.trelby.org/assets/manual.html