Answering my own question, I finally used a solution with HTML download attribute, nicely described here. When this technique is used in Electron, it presents a file save dialog which is exactly what I want. When used in a browser, this technique just downloads the file without a prompt, so I will continue using File System Access API for browser environments.

Here is the code that handles downloading when running in Electron.

function download(filename: string, contents: string) {
    var element = document.createElement('a');
    element.setAttribute('href', 'data:text/plain;charset=utf-8,' + encodeURIComponent(contents));
    element.setAttribute('download', filename); = 'none';


let isElectron = require("is-electron");
export async function chooseAndWriteToFileUniversal(contents: string) {
    if (isElectron()) {
        download("data.txt", contents)
    } else {
        chooseAndWriteToFile(contents) // See the original question for implementation of this function

Still, would be nice to know why/how is Chromium in Electron more restricted than in a normal Chrome or Edge browser, and if it can be changed.

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