XFA - Add <a> element in button when an url is detected (bug 1716758)

- it aims to fix https://bugzilla.mozilla.org/show_bug.cgi?id=1716758;
  - some buttons have a JS action with the pattern `app.launchURL(...)` (or similar) so extract when it's possible the url and generate a <a> element with the href equals to the found url;
  - pdf.js already had some code to handle that so this patch slightly refactor that.

src/core/xfa/template.js

@@ -76,6 +76,7 @@ import {
   createWrapper,
   fixDimensions,
   fixTextIndent,
+  fixURL,
   isPrintOnly,
   layoutClass,
   layoutNode,
@@ -100,6 +101,7 @@ import {
 } from "./utils.js";
 import { stringToBytes, Util, warn } from "../../shared/util.js";
 import { getMetrics } from "./fonts.js";
+import { recoverJsURL } from "../core_utils.js";
 import { searchNode } from "./som.js";
 
 const TEMPLATE_NS_ID = NamespaceIds.template.id;
@@ -1066,7 +1068,10 @@ class Button extends XFAObject {
 
   [$toHTML](availableSpace) {
     // TODO: highlight.
-    return HTMLResult.success({
+
+    const parent = this[$getParent]();
+    const grandpa = parent[$getParent]();
+    const htmlButton = {
       name: "button",
       attributes: {
         id: this[$uid],
@@ -1074,7 +1079,38 @@ class Button extends XFAObject {
         style: {},
       },
       children: [],
-    });
+    };
+
+    for (const event of grandpa.event.children) {
+      // if (true) break;
+      if (event.activity !== "click" || !event.script) {
+        continue;
+      }
+      const jsURL = recoverJsURL(event.script[$content]);
+      if (!jsURL) {
+        continue;
+      }
+      const href = fixURL(jsURL.url);
+      if (!href) {
+        continue;
+      }
+      const target = jsURL.newWindow ? "_blank" : undefined;
+
+      // we've an url so generate a <a>
+      htmlButton.children.push({
+        name: "a",
+        attributes: {
+          id: "link" + this[$uid],
+          href,
+          target,
+          class: ["xfaLink"],
+          style: {},
+        },
+        children: [],
+      });
+    }
+
+    return HTMLResult.success(htmlButton);
   }
 }
 
@@ -2897,7 +2933,12 @@ class Field extends XFAObject {
       ui.attributes.style = Object.create(null);
     }
 
+    let aElement = null;
+
     if (this.ui.button) {
+      if (ui.children.length === 1) {
+        [aElement] = ui.children.splice(0, 1);
+      }
       Object.assign(ui.attributes.style, borderStyle);
     } else {
       Object.assign(style, borderStyle);
@@ -2955,6 +2996,10 @@ class Field extends XFAObject {
       }
     }
 
+    if (aElement) {
+      ui.children.push(aElement);
+    }
+
     if (!caption) {
       if (ui.attributes.class) {
         // Even if no caption this class will help to center the ui.