Na página de extensão do StumbleUpon , eles afirmam:
Chrome does not allow DOM injection into some pages so the toolbar is not visible on those pages (including chrome product URLs like the extensions page, and newly opened tabs)
Isso está de acordo com os documentos do desenvolvedor do Chrome. Nas Perguntas frequentes sobre o desenvolvedor de extensão :
Can I add a content script to
chrome://URLs?No. The extensions APIs have been designed to minimize backwards compatibility issues that can arise when new versions of the browser are pushed. Allowing content scripts on
chrome://URLs would mean that developers would begin to rely on the DOM, CSS, and JavaScript of these pages to stay the same. In the best case, these pages could not be updated as quickly as they are being updated right now. In the worst case, it could mean that an update to one of these pages could cause an extension to break, causing key parts of the browser to stop working for users of that extension.The reason that replacing the content hosted at these URLs entirely is allowed is because it forces an extension developer to implement all of the functionality they want without depending on the browser's internal implementation to stay the same.
Basicamente, com a API de Extensões atual do Chrome, a única maneira de o StumbleUpon colocar sua barra na nova página de guia seria substituir completamente a página da guia atual, o que significaria que eles teriam que escrever os seus. Isso também significaria que caberia ao StumbleUpon garantir que os aplicativos e extensões recém-instalados fossem exibidos na página, etc., o que é muito mais trabalhoso do que eles provavelmente desejam.