Bug 1866884 - Make browser_alternate_fixup_middle_click_link.js wait for error page properly. r=Gijs

Depends on D198571

Differential Revision: https://phabricator.services.mozilla.com/D198574

Author: emilio

docshell/test/browser/browser_alternate_fixup_middle_click_link.js

@@ -16,27 +16,20 @@ add_task(async function test_alt_fixup_middle_click() {
       link.textContent = "Me, me, click me!";
       content.document.body.append(link);
     });
-    let newTabPromise = BrowserTestUtils.waitForNewTab(gBrowser);
+    let newTabPromise = BrowserTestUtils.waitForNewTab(
+      gBrowser,
+      /* wantLoad = */ null,
+      /* waitForLoad = */ true,
+      /* waitForAnyTab = */ false,
+      /* maybeErrorPage = */ true
+    );
     await BrowserTestUtils.synthesizeMouseAtCenter(
       "a[href]",
       { button: 1 },
       browser
     );
     let tab = await newTabPromise;
     let { browsingContext } = tab.linkedBrowser;
-    // Account for the possibility of a race, where the error page has already loaded:
-    if (
-      !browsingContext.currentWindowGlobal?.documentURI.spec.startsWith(
-        "about:neterror"
-      )
-    ) {
-      await BrowserTestUtils.browserLoaded(
-        tab.linkedBrowser,
-        false,
-        null,
-        true
-      );
-    }
     // TBH, if the test fails, we probably force-crash because we try to reach
     // *www.* example.com, which isn't proxied by the test infrastructure so
     // will forcibly abort the test. But we need some asserts so they might as

testing/mochitest/BrowserTestUtils/BrowserTestUtils.sys.mjs

@@ -738,6 +738,8 @@ export var BrowserTestUtils = {
    * @param {boolean} [waitForAnyTab = false]
    *        True to wait for the url to be loaded in any new tab, not just the next
    *        one opened.
+   * @param {boolean} [maybeErrorPage = false]
+   *        See ``browserLoaded`` function.
    *
    * @return {Promise}
    * @resolves With the {xul:tab} when a tab is opened and its location changes
@@ -750,7 +752,8 @@ export var BrowserTestUtils = {
     tabbrowser,
     wantLoad = null,
     waitForLoad = false,
-    waitForAnyTab = false
+    waitForAnyTab = false,
+    maybeErrorPage = false
   ) {
     let urlMatches;
     if (wantLoad && typeof wantLoad == "function") {
@@ -779,7 +782,8 @@ export var BrowserTestUtils = {
             result = BrowserTestUtils.browserLoaded(
               newBrowser,
               false,
-              urlMatches
+              urlMatches,
+              maybeErrorPage
             ).then(() => newTab);
           } else {
             // If not waiting for load, just resolve with the new tab.