David Ponce <da_vid@orange.fr> writes:

> 1st line with a gray pressed-button box around the text. Work as expected.
> 2nd line with a gray released-button box around the text. Work as expected.
> 3rd line with a gray flat-button box around the text. Doesn' work as
> expected. The gray line around the text is not visible.

With a flat button, there is no line around the button.  So you'd
typically say

 (propertize "This buffer is for text"
	     'font-lock-face
	     '( :box ( :line-width (4 . 4)
		       :style flat-button)
	      :background "dark grey"))


and get