Merge "Special:PrefixIndex: Fix regression on prefix input value"

diff --cc includes/specials/SpecialPrefixindex.php
index fac71b2,b07869c..2f285c9
--- 1/includes/specials/SpecialPrefixindex.php
--- 2/includes/specials/SpecialPrefixindex.php
+++ b/includes/specials/SpecialPrefixindex.php
@@@ -124,10 -126,12 +126,12 @@@ class SpecialPrefixindex extends Specia
                                'label-message' => 'prefixindex-strip',
                        ],
                ];
-               $htmlForm = new HTMLForm( $formDescriptor, $this->getContext() );
+               $context = new DerivativeContext( $this->getContext() );
+               $context->setTitle( $this->getPageTitle() ); // Remove subpage
+               $htmlForm = new HTMLForm( $formDescriptor, $context );
                $htmlForm
                        ->setMethod( 'get' )
 -                      ->setWrapperLegendMsg( 'allpages' )
 +                      ->setWrapperLegendMsg( 'prefixindex' )
                        ->setSubmitTextMsg( 'prefixindex-submit' );
  
                return $htmlForm->prepareForm()->getHTML( false );