[5.x] Allow selecting entries from different sites within the link fieldtype

src/Fieldtypes/Link.php

@@ -37,6 +37,11 @@ protected function configFieldItems(): array
                         'mode' => 'select',
                         'max_items' => 1,
                     ],
+                    'select_across_sites' => [
+                        'display' => __('Select Across Sites'),
+                        'instructions' => __('statamic::fieldtypes.entries.config.select_across_sites'),
+                        'type' => 'toggle',
+                    ],
                 ],
             ],
         ];
@@ -47,7 +52,8 @@ public function augment($value)
         return new ArrayableLink(
             $value
                 ? ResolveRedirect::item($value, $this->field->parent(), true)
-                : null
+                : null,
+            ['select_across_sites' => $this->canSelectAcrossSites()]
         );
     }
 
@@ -108,6 +114,7 @@ private function nestedEntriesFieldtype($value): Fieldtype
             'type' => 'entries',
             'max_items' => 1,
             'create' => false,
+            'select_across_sites' => $this->canSelectAcrossSites(),
         ]));
 
         $entryField->setValue($value);
@@ -190,4 +197,31 @@ public function toGqlType()
             },
         ];
     }
+
+    protected function getConfiguredCollections()
+    {
+        return empty($collections = $this->config('collections'))
+            ? \Statamic\Facades\Collection::handles()->all()
+            : $collections;
+    }
+
+    private function canSelectAcrossSites(): bool
+    {
+        return $this->config('select_across_sites', false);
+    }
+
+    private function availableSites()
+    {
+        if (! Site::hasMultiple()) {
+            return [];
+        }
+
+        $configuredSites = collect($this->getConfiguredCollections())->flatMap(fn ($collection) => \Statamic\Facades\Collection::find($collection)->sites());
+
+        return Site::authorized()
+            ->when(isset($configuredSites), fn ($sites) => $sites->filter(fn ($site) => $configuredSites->contains($site->handle())))
+            ->map->handle()
+            ->values()
+            ->all();
+    }
 }

src/Fieldtypes/Link/ArrayableLink.php

@@ -2,6 +2,7 @@
 
 namespace Statamic\Fieldtypes\Link;
 
+use Illuminate\Support\Arr;
 use Statamic\Fields\ArrayableString;
 
 class ArrayableLink extends ArrayableString
@@ -26,6 +27,14 @@ public function jsonSerialize()
 
     public function url()
     {
-        return is_object($this->value) ? $this->value?->url() : $this->value;
+        if (! is_object($this->value)) {
+            return $this->value;
+        }
+
+        if (Arr::get($this->extra(), 'select_across_sites')) {
+            return $this->value->absoluteUrl();
+        }
+
+        return $this->value?->url();
     }
 }