I wonder if some setting got inadvertently changed, because I could have
sworn that I used to hit "shift + home" in order to highlight a single row
from right to left. Now, the entire worksheet highlights all the rest of the
way to the top, & to the left from the cell I'm in.

Let's say I'm over in cell K20. I want to highlight back to column A20,
just row 20. I hit "shift + home," and the whole worksheet now highlights
from K20 to A1.

Someone else worked on my spreadsheet, & I'm wondering if something got
changed.