The earlier question about typing macrons does not discuss adding a breve.
My solution using Autohotkey works for any Unicode character (or basically any action you'd like to take on your computer), so also for brevia. I just type whatever combination or sequence of keys I have chosen to get a breve. I have set a\/
for ă, as it is easy to remember. (You can guess how I type â.) No modifier keys, no combinations that are hard to remember.
The Autohotkey code required is as follows, to be changed at your pleasure:
You can basically add any character you like by copy-pasting the last line and copy-pasting a Unicode character you have found on e.g. Wikipaedia instead of ў, and typing some easy-to-remember key sequence instead of y\/
. That's how I usually add new characters. But there are even easier ways (ask me).
When I'm writing Latin I usually use the ABC Extended keyboard layout (in System Preferences > Keyboard > Input Sources), as @Undo suggested in the answers to the linked question. In that layout, Option-B or Alt-B (depending on your OS) adds a breve to whatever vowel you type next.
(I wish I had @Undo's facility with display so that I could make this answer show exactly how this worked, but, alas, I am merely mortal.)
You can use html entities, but they are less convenient than for macrons:
For macrons there is an alternative way that is easier to remember (ā is ā
), but for a breve the only option seems to be to use numbers.
The Y in the list above is a bit of a hack: I took the lower case y from the Cyrillic and upper case Y from the Greek alphabet. I did not find a Y with breve for the Latin alphabet in the html entities.
Another option is to simply copy from the list above.
You can add a breve to any letter with ̆
This leaves you with only one entity code to remember and it can be easily combined with any character.
(Suggested by Mr Lister in comments.)