edge_fullscreen/Extension/js/popup.js

// get the buttons by id
let start = document.getElementById('btn_start');
let stop = document.getElementById('btn_stop');

start.onclick = function() {
	browser.windows.update(browser.windows.WINDOW_ID_CURRENT,{state:"fullscreen"});
	console.log("test start！！！！");
};

stop.onclick = function() {
	browser.windows.update(browser.windows.WINDOW_ID_CURRENT,{state:"normal"});
	console.log("test stop！！！！");
};