bug#73022: 31.0.50; Crash in build_frame_matrix_from_leaf_window after C-x 2 and reducing terminal size

[Bug reports for GNU Emacs, the Swiss army knife of text editors]

One detail: If looking at the fourth, final screenshot I sent earlier,
then I notice that the last character of the lower mode line is black
which seems to correspond to curX (tty) == FrameCols (tty).  Does this
mean that the cursor is on the mode line of the lower normal window?

martin