diff --git a/src/Control/Session.php b/src/Control/Session.php
index a5d904468..f0761c80e 100644
--- a/src/Control/Session.php
+++ b/src/Control/Session.php
@@ -323,6 +323,12 @@ class Session
                 }
 
                 session_start();
+
+                // Session start emits a cookie, but only if there's no existing session. If there is a session timeout
+                // tied to this request, make sure the session is held for the entire timeout by refreshing the cookie age.
+                if ($timeout && $this->requestContainsSessionId($request)) {
+                    Cookie::set(session_name(), session_id(), $timeout / 86400, $path, $domain ?: null, $secure, true);
+                }
             } else {
                 // If headers are sent then we can't have a session_cache_limiter otherwise we'll get a warning
                 session_cache_limiter(null);
diff --git a/tests/php/Forms/TreeMultiselectFieldTest.php b/tests/php/Forms/TreeMultiselectFieldTest.php
index 5b2f14a65..e2cc56741 100644
--- a/tests/php/Forms/TreeMultiselectFieldTest.php
+++ b/tests/php/Forms/TreeMultiselectFieldTest.php
@@ -7,11 +7,17 @@ use SilverStripe\Dev\SapphireTest;
 use SilverStripe\Forms\Form;
 use SilverStripe\Forms\FormTemplateHelper;
 use SilverStripe\Forms\TreeMultiselectField;
+use SilverStripe\ORM\Tests\HierarchyTest\TestObject;
+use SilverStripe\View\SSViewer;
 
 class TreeMultiselectFieldTest extends SapphireTest
 {
     protected static $fixture_file = 'TreeDropdownFieldTest.yml';
 
+    protected static $extra_dataobjects = [
+        TestObject::class,
+    ];
+
     protected $formId = 'TheFormID';
     protected $fieldName = 'TestTree';
 
@@ -54,6 +60,9 @@ class TreeMultiselectFieldTest extends SapphireTest
     {
         parent::setUp();
 
+        // Don't let other themes interfere with these tests
+        SSViewer::set_themes([]);
+
         $this->form = $this->buildFormMock();
         $this->field = $this->buildField($this->form);
         $this->folders = $this->loadFolders();