"Add period with double space." Apple knows what's up: people who add only a single space after a period should be given 40 lashes with a wet noodle.
Even with proportional fonts, a single space after a sentence rather than two spaces makes the text harder to read. It looks run on. The period is a FULL stop, not a quick, breathless rush to the next sentence!Far too many text entry dialog boxes take one's nicely formatted text, with two spaces after a period (or question mark) and convert them into only one sentence, thereby, making the comment harder to read!
There aren't many things in grammar that I can point to as being absolutely write or wrong....