core/lib/Drupal/Core/Render/RenderCache.php | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/core/lib/Drupal/Core/Render/RenderCache.php b/core/lib/Drupal/Core/Render/RenderCache.php
index e2b0276..1a8339b 100644
--- a/core/lib/Drupal/Core/Render/RenderCache.php
+++ b/core/lib/Drupal/Core/Render/RenderCache.php
@@ -215,7 +215,15 @@ public function set(array &$elements, array $pre_bubbling_elements) {
       }
 
       // Calculate the union of the cacheability for this request and the
-      // current (stored) redirecting cache item.
+      // current (stored) redirecting cache item. We need:
+      // - the union of cache contexts, because that is how we know which cache
+      //   item to redirect to;
+      // - the union of cache tags, because that is how we know when the cache
+      //   redirect cache item itself is invalidated;
+      // - the union of max ages, because that is how we know when the cache
+      //   redirect cache item itself becomes stale. (Without this, we might end
+      //   up toggling between a permanently and a briefly cacheable cache
+      //   redirect, because the last update's max-age would always "win".)
       $redirect_cacheability_updated = CacheableMetadata::createFromRenderArray($data)->merge($redirect_cacheability);
 
       // Stored cache contexts incomplete: this request causes cache contexts to