Perhaps I am expecting too much, but the Mirror tool should truly mirror the condition of a line.
Note the Joined icon on left.
Note no Joined point on right. Should be…
Ok, I could have drawn the triangle but in the more general case say the horizontal line was longer and I really don’t know where to draw the line for the same triangle? And say I can’t depend on the grid. Mirror should give me an accurate closed face… How else?