Function: truncated-partial-width-window-p

Return non-nil if lines in WINDOW are specifically truncated due to its width.
WINDOW must be a live window and defaults to the selected one.
Return nil if WINDOW is not a partial-width window
(regardless of the value of `truncate-lines').
Otherwise, consult the value of `truncate-partial-width-windows'
for the buffer shown in WINDOW.