PR #3744 and #3765 introduced contentCursorRow which diverges from the
actual terminal cursor position after IME repositioning. computeLineDiff
computes ANSI escape movements which are relative to where the cursor
physically is — that must be hardwareCursorRow, not a phantom position.
Remove contentCursorRow entirely and revert computeLineDiff baseline to
hardwareCursorRow. The ghost-line test was asserting wrong movement
direction (UP from phantom position vs DOWN from actual cursor).
Closes#3764
c0236254a2