Confirm messages were broken in Firefox 67

For #271

lib/modal.css

@@ -0,0 +1,38 @@
+.modal {
+	position: fixed;
+	top: 0;
+	left: 0;
+	bottom: 0;
+	right: 0;
+	text-align: center;
+	z-index: 100;
+}
+
+.modal .overlay {
+	position: absolute;
+	top: 0;
+	left: 0;
+	bottom: 0;
+	right: 0;
+	background-color: rgba(255, 255, 255, 0.35);
+}
+
+.modal .dialogPosition {
+	position: absolute;
+	top: 10%;
+	width: 100%;
+}
+
+.modal .dialog {
+	display: block;
+	margin: 0 auto;
+	padding: 1em;
+	max-width: 300px;
+	min-width: 200px;
+	background-color: var(--background-color);
+	border: 1px solid currentColor;
+}
+
+.modal .buttons {
+	margin-top: 1em;
+}

lib/modal.js

@@ -0,0 +1,131 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+(function(){
+	"use strict";
+	
+	let scope;
+	if ((typeof exports) !== "undefined"){
+		scope = exports;
+	}
+	else {
+		scope = require.register("./modal", {});
+	}
+	
+	const extension = require("./extension");
+	
+	function getGlobalOffsetTop(node){
+		if (node){
+			return node.offsetTop + getGlobalOffsetTop(node.offsetParent);
+		}
+		else {
+			return 0;
+		}
+	}
+	function getGlobalScrollTop(node){
+		if (node && node.scrollTop){
+			return node.scrollTop + getGlobalScrollTop(node.parentNode);
+		}
+		else {
+			return window.scrollY;
+		}
+	}
+	
+	function openDialog(text, buttons, parent = document.body){
+		if (!(parent instanceof Node)){
+			const parentSelector = parent.selector;
+			parent = parent.node;
+			while (parent && !parent.matches(parentSelector)){
+				parent = parent.parentNode;
+			}
+			if (!parent){
+				parent = document.body;
+			}
+		}
+		const container = document.createElement("div");
+		container.className = "modal";
+		parent.appendChild(container);
+		
+		const overlay = document.createElement("div");
+		overlay.className = "overlay";
+		container.appendChild(overlay);
+		
+		const dialogPosition = document.createElement("div");
+		dialogPosition.className = "dialogPosition";
+		container.appendChild(dialogPosition);
+		
+		const dialog = document.createElement("div");
+		dialog.className = "dialog";
+		dialogPosition.appendChild(dialog);
+		
+		const textNode = document.createElement("span");
+		textNode.className = "text";
+		textNode.textContent = text;
+		dialog.appendChild(textNode);
+		
+		const buttonsNode = document.createElement("div");
+		buttonsNode.className = "buttons";
+		dialog.appendChild(buttonsNode);
+		
+		let defaultButton;
+		buttons.forEach(function(button){
+			const buttonNode = document.createElement("button");
+			buttonNode.textContent = button.text;
+			buttonNode.addEventListener("click", function(){
+				close();
+				button.callback();
+			});
+			buttonsNode.appendChild(buttonNode);
+			if (button.focused){
+				buttonNode.focus();
+			}
+			if (button.default){
+				defaultButton = button;
+			}
+		});
+		
+		function closeOnEscape(event){
+			if (event.keyCode === 27){
+				close();
+				if (defaultButton){
+					defaultButton.callback();
+				}
+			}
+		}
+		function positionDialog(){
+			const parentTop = getGlobalOffsetTop(parent) - getGlobalScrollTop(parent);
+			const parentHeight = parent.offsetHeight;
+			const height = dialog.offsetHeight;
+			const top = Math.max(
+				0,
+				Math.min(
+					container.offsetHeight - height,
+					parentTop + parentHeight / 2 - height / 2
+				)
+			);
+			dialogPosition.style.top = top + "px";
+		}
+		function close(){
+			window.removeEventListener("keydown", closeOnEscape);
+			window.removeEventListener("scroll", positionDialog);
+			window.removeEventListener("resize", positionDialog);
+			parent.removeChild(container);
+		}
+		window.addEventListener("keydown", closeOnEscape);
+		
+		if (parent !== document.body){
+			positionDialog();
+			window.addEventListener("scroll", positionDialog);
+			window.addEventListener("resize", positionDialog);
+		}
+	}
+	
+	scope.confirm = function(text, parent){
+		return new Promise(function(resolve){
+			openDialog(text, [
+				{text: extension.getTranslation("cancel"), default: true, callback: ()=>resolve(false)},
+				{text: extension.getTranslation("OK"), focused: true, callback: ()=>resolve(true)}
+			], parent);
+		});
+	};
+}());