Features missing, developers helping, users installing :/
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Edited:

Oops, doesn't work now, no any button shows on Readme.md header at repo tree page.

Conflict with Refined Github, with whose features 'hide-readme-header' and 'remove-upload-files-button' disabled, ToC button shows on Readme.md header at blob page. However, its popover list isn't clickable unless I disable Refine Github extension.

But even if with Refined Github WebExtension ITSELF disabled there's not ToC button at repo tree page.