Fix considering "same domain-ish" URLs as internal (PR #1126)

In 6b266b7815, as I got rid of deprecated dep `wurl`, I wrote:

> This one may be problematic, as it used to do TLD stuff:
> https://github.com/websanova/node-url/blob/7982a613bc/wurl.js#L4
>
> So, the new WHATWG-URL-based implementation will consider
> `asana.com` to be "external" to `app.asana.com`, contrarily to before.
> Given the nature of Nativefier, I think it's actually what to expect,
> that in this case you're "out of the app", and in e.g. asana's landing
> page, which you'd expect to see in your browser.

Turns out it's even more problematic: @TheCleric notices in https://github.com/nativefier/nativefier/pull/1124#issuecomment-790279403
that this breaks app `https://evernote.com` doing its login in `www.evernote.com`

The present change fixes this, by behaving mostly similarly to before,
but without re-introducing `wurl` or another dep needing a TLD/SLD list.

app/src/helpers/helpers.test.ts

@@ -1,6 +1,10 @@
 import { linkIsInternal, getCounterValue } from './helpers';
 
 const internalUrl = 'https://medium.com/';
+const internalUrlWww = 'https://www.medium.com/';
+const sameBaseDomainUrl = 'https://app.medium.com/';
+const internalUrlCoUk = 'https://medium.co.uk/';
+const sameBaseDomainUrlCoUk = 'https://app.medium.co.uk/';
 const internalUrlSubPath = 'topic/technology';
 const externalUrl = 'https://www.wikipedia.org/wiki/Electron';
 const wildcardRegex = /.*/;
@@ -27,6 +31,28 @@ test('all urls should be internal with wildcard regex', () => {
   expect(linkIsInternal(internalUrl, externalUrl, wildcardRegex)).toEqual(true);
 });
 
+test('a "www." of a domain should be considered internal', () => {
+  expect(linkIsInternal(internalUrl, internalUrlWww, undefined)).toEqual(true);
+});
+
+test('urls on the same "base domain" should be considered internal', () => {
+  expect(linkIsInternal(internalUrl, sameBaseDomainUrl, undefined)).toEqual(
+    true,
+  );
+});
+
+test('urls on the same "base domain" should be considered internal, even with a www', () => {
+  expect(linkIsInternal(internalUrlWww, sameBaseDomainUrl, undefined)).toEqual(
+    true,
+  );
+});
+
+test('urls on the same "base domain" should be considered internal, long SLD', () => {
+  expect(
+    linkIsInternal(internalUrlCoUk, sameBaseDomainUrlCoUk, undefined),
+  ).toEqual(true);
+});
+
 const smallCounterTitle = 'Inbox (11) - nobody@example.com - Gmail';
 const largeCounterTitle = 'Inbox (8,756) - nobody@example.com - Gmail';
 const noCounterTitle = 'Inbox - nobody@example.com - Gmail';

app/src/helpers/helpers.ts

@@ -33,9 +33,17 @@ export function linkIsInternal(
   }
 
   try {
-    const currentDomain = new URL(currentUrl).hostname;
-    const newDomain = new URL(newUrl).hostname;
-    return currentDomain === newDomain;
+    // Consider as "same domain-ish", without TLD/SLD list:
+    // 1. app.foo.com and foo.com
+    // 2. www.foo.com and foo.com
+    // 3. www.foo.com and app.foo.com
+    const currentDomain = new URL(currentUrl).hostname.replace(/^www\./, '');
+    const newDomain = new URL(newUrl).hostname.replace(/^www./, '');
+    const [longerDomain, shorterDomain] =
+      currentDomain.length > newDomain.length
+        ? [currentDomain, newDomain]
+        : [newDomain, currentDomain];
+    return longerDomain.endsWith(shorterDomain);
   } catch (err) {
     console.warn(
       'Failed to parse domains as determining if link is internal. From:',

docs/api.md

@@ -400,7 +400,13 @@ Forces the packaged app to ignore web security errors, such as [Mixed Content](h
 --internal-urls <regex>
 ```
 
-Regular expression of URLs to consider "internal"; all other URLs will be opened in an external browser. Defaults to URLs on same second-level domain as app.
+Regular expression of URLs to consider "internal" while following a hyperlink.
+Internal URLs will open in Nativefier, other URLs will open in your preferred browser.
+
+Defaults to view as "internal" two URLs that share the same base domain,
+once stripped of `www.`. For example, by default,
+- URLs from/to `foo.com`, `app.foo.com`, `www.foo.com` are considered internal.
+- URLs from/to `abc.com` and `xyz.com` are considered external.
 
 Example: