Bug 1667726 - Clean up custom controls when leaving Welcome page. r=mhowell

Differential Revision: https://phabricator.services.mozilla.com/D93706

browser/installer/windows/nsis/uninstaller.nsi

@@ -637,6 +637,10 @@ Function un.preWelcome
   ShowWindow $0 ${SW_HIDE}
 
   ${If} $ShouldPromptForRefresh == "1"
+    ; Note: INI strings added here (rather than overwriting an existing value)
+    ; should be removed in un.leaveWelcome, since ioSpecial.ini is reused
+    ; for the Finish page.
+
     ; Replace title and body text
     WriteINIStr "$PLUGINSDIR\ioSpecial.ini" "Field 2" Text "$(UN_REFRESH_PAGE_TITLE)"
     ; Convert to translate newlines, this includes $PLUGINSDIR internally.
@@ -749,6 +753,13 @@ Function un.leaveWelcome
     Quit
   ${EndIf}
 
+  ${If} $ShouldPromptForRefresh == "1"
+    ; Remove the custom controls.
+    WriteINIStr "$PLUGINSDIR\ioSpecial.ini" "Settings" NumFields 3
+    DeleteIniSec "$PLUGINSDIR\ioSpecial.ini" "Field 4"
+    DeleteIniSec "$PLUGINSDIR\ioSpecial.ini" "Field 5"
+  ${EndIf}
+
   ; Bring back the header bitmap for the next pages.
   GetDlgItem $0 $HWNDPARENT 1046
   ShowWindow $0 ${SW_SHOW}