Cannot close chat pop-up on iPhone

When the facebook chat plugin pops up on an iPhone, the screen displayed is too big to see the “minimise” button or icon in the right upper corner. This means that the user cannot get rid of the pop-up screen, and the website itself is hidden behind the pop-up screen, which is very annoying from a UX perspective.

How can the pop-up screen be made smaller on an iPhone, so that a user can minimise or remove the pop-up screen?

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