truncate-partial-width-windows

truncate-partial-width-windows is a variable defined in `C source code'.
Its value is

50

Documentation:
Non-nil means truncate lines in windows narrower than the frame.
For an integer value, truncate lines in each window narrower than the
full frame width, provided the window width is less than that integer;
otherwise, respect the value of `truncate-lines'.

For any other non-nil value, truncate lines in all windows that do
not span the full frame width.

A value of nil means to respect the value of `truncate-lines'.

If `word-wrap' is enabled, you might want to reduce this.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.