Always generate action URLs from the base CP URL in headless mode

#5553
https://github.com/craftcms/cms/issues/6597#issuecomment-759146497

CHANGELOG-v3.6.md

@@ -132,6 +132,7 @@
 - Field layout designers will no longer create a new tab if no tab name is entered in the prompt. ([#7333](https://github.com/craftcms/cms/issues/7333))
 - Site URLs that are generated on the front-end of disabled sites now include the `siteToken` param, if one was passed to the current page. ([#7264](https://github.com/craftcms/cms/issues/7264))
 - Mutex lock names are now prefixed with the application ID, to avoid lock conflicts if two Craft installs shared the same database. ([#7384](https://github.com/craftcms/cms/issues/7384))
+- Action URLs are now always based on the control panel URL when running Craft in headless mode. ([#5553](https://github.com/craftcms/cms/issues/5553))
 - Renamed the `backup` and `restore` commands to `db/backup` and `db/restore`. ([#7023](https://github.com/craftcms/cms/issues/7023))
 - The `migrate/all` command now lists the migrations that will be applied. ([#7381](https://github.com/craftcms/cms/issues/7381))
 - The `project-config/apply` command now displays a list of changes it is applying. ([#7235](https://github.com/craftcms/cms/issues/7235))

CHANGELOG.md

@@ -36,6 +36,7 @@
 - Field containers no longer set the `aria-describedby` attribute, leaving it up to the actual inputs to do so. ([#7365](https://github.com/craftcms/cms/issues/7365))
 - Number field settings and input values are now fully formatted, unless the Preview Format setting is set to “Unformatted”.
 - Mutex lock names are now prefixed with the application ID, to avoid lock conflicts if two Craft installs shared the same database. ([#7384](https://github.com/craftcms/cms/issues/7384))
+- Action URLs are now always based on the control panel URL when running Craft in headless mode. ([#5553](https://github.com/craftcms/cms/issues/5553))
 - The `migrate/all` command now lists the migrations that will be applied. ([#7381](https://github.com/craftcms/cms/issues/7381))
 - The `project-config/apply` command now displays a list of changes it is applying. ([#7235](https://github.com/craftcms/cms/issues/7235))
 - The `allowedFileExtensions` config setting now includes several file extensions used by caption and subtitle file formats by default. ([#7304](https://github.com/craftcms/cms/issues/7304))

src/helpers/UrlHelper.php

@@ -342,7 +342,7 @@ class UrlHelper
 
         $request = Craft::$app->getRequest();
 
-        if ($request->getIsCpRequest()) {
+        if ($generalConfig->headlessMode || $request->getIsCpRequest()) {
             $path = static::prependCpTrigger($path);
             $cpUrl = true;
         } else {