Because this image always appears to me. I don't know if there is a graphics problem, or a software error. Thank you.
It´s a bug. It occurs for example, if you try to edit invisible elements or invisible text-elements.
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There is a bug that @ktewes mentioned where an overzealous selection can take over the screen. Usually selecting text will undo the problem.
Otherwise it is hard to tell from your screenshot, but note that entering symbols will dim out other elements. This behavior can be controlled via the visibility toggle: 
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where is this image? I have not found...
hope you'll find it without marking 
Gracias!!