First, I would personally change the title to “The Heart of the Matter, a Matter of Fact.”

                “A matter of the heart” makes your relationship with your grandmother seem overly semimetal and bitter, based on the ending. A “matter of fact” makes it more punchy.

                Overall, I liked this one a lot. I think you are definitely giving words breathing room where they need it, without being too gimmicky with the hanging words.

                Visually, the first two stanzas are beautiful (especially the second one). I love “on a screen considerably / smaller” The word smaller is dwarfed by the line above it, and still smaller than the line after it, so it gives a visual feel for the meaning of the word.

                Also, “she watches her game-shows / on.” is very nice. I love how it picks up the word “on” and places it on another line. It’s another instance of having the word visually do what you want it to mean.

                The third stanza doesn’t hold together as well as the other two. The phrase “that as many / windows as I / may open,” seems too wordy. I don’t know how to do it, but I want you to be able to say that whole thing simply in one line. This stanza also doesn’t have the nice weaving visual flow as the first two stanzas. Squint your eyes at the poem, and you’ll see there is something “off” about that stanza. It’s stubby.

                I think, because you were so emotionally involved with this stanza, you stumbled over it. You are trying to give us this witty synopses here, and it means a lot, but because it means so much, it doesn’t come out just right.

                Putting “she slams” on its own line seems unnecessary. I want to be left thinking about the metaphor of slammed doors and open windows, instead of “Damn, this guy is bitter.” I want it to end quietly and reflective… but that may not be what you want, so take it or leave it.