FIX: Readonly transformation of lazy-loaded searchable dropdown

src/Forms/LazyLoadedLookupField.php

@@ -0,0 +1,51 @@
+<?php
+
+namespace SilverStripe\Forms;
+
+use SilverStripe\Core\Convert;
+use SilverStripe\ORM\ArrayLib;
+use SilverStripe\ORM\DataObjectInterface;
+use SilverStripe\ORM\FieldType\DBField;
+use SilverStripe\ORM\ArrayList;
+use SilverStripe\ORM\DataList;
+
+/**
+ * Read-only complement of {@link SearchableDropdownField} and {@link SearchableMultiDropdownField}.
+ *
+ * Shows the "human value" of the SearchableDropdownField for the currently selected
+ * values.
+ */
+class LazyLoadedLookupField extends LookupField
+{
+    private ?DataList $sourceList = null;
+
+    /**
+    * To retain compatibility with ancestor getSource() this returns an array of only the selected values
+    */
+    public function getSource(): array
+    {
+        $values = $this->getValueArray();
+        if (empty($values) || $this->sourceList === null) {
+            $selectedValuesList = ArrayList::create();
+        } else {
+            $selectedValuesList = $this->sourceList->filterAny(['ID' => $values]);
+        }
+        return $this->getListMap($selectedValuesList);
+    }
+
+    /*
+    * @param mixed $source
+    */
+    public function setSource($source): static
+    {
+        // Setting to $this->sourceList instead of $this->source because SelectField.source
+        // docblock type is array|ArrayAccess i.e. does not allow DataList
+
+        if ($source instanceof DataList) {
+            $this->sourceList = $source;
+        } else {
+            $this->sourceList = null;
+        }
+        return $this;
+    }
+}

src/Forms/SearchableDropdownTrait.php

@@ -586,4 +586,13 @@ private function updateOptionsForSchema(
         }
         return $options;
     }
+
+    public function performReadonlyTransformation()
+    {
+        $field = FormField::castedCopy(LazyLoadedLookupField::class);
+        $field->setSource($this->sourceList);
+        $field->setReadonly(true);
+
+        return $field;
+    }
 }