I had thought of a idea in a operating system design, which has many similarities to this but also many significant differences. The rest of the operating system would also be designed to support this, and the REPL would also be integrated with editors (like they show) and with a zoned spreadsheet. There would also be transactions (not the same as described there, but there is a similarity), and also capabilities (usable for security and many other things), and possibility to move objects between windows (this also allows it to work better with GUI; e.g. you can write a loop and use some combination of keys with mouse to drag a command button from a GUI window into your text in order to execute that command button within the loop; or you can copy data displayed in a table in another program in order to perform calculations on it or write a conversion to other format to then copy into another program, etc). However, tabs and panes would be handled by the window manager. However, it would use a binary format (a kind of TLV format), and it would not use CSS and HTML and Unicode. For rich text and other things there would also be a Reveal Codes function in order to reveal (and modify) the codes.
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