How to auto-lock MacBook when inactive (without enabling screensaver)
You could try doing this:


Turn screensaver off → System Preferences &gt; Desktop &amp; Screen Saver (Screen Saver tab) &gt; Start after: never (bottom left).
Make sure the mac will go to sleep automatically → System Preferences &gt; Energy Saver → Turn display off after.... Both computer sleep and dispay sleep will lock it.



  For the mac to auto lock you of course need System Preferences &gt; Security &amp; Privacy &gt; General (1st tab) &gt; Require password ____ after sleep or screen saver begins (checkbox) to be checked.


I didn't do very extensive testing, but after putting the mac to sleep or just the display and then unlocking the mac, it seems to let me write in the Chrome textarea... just with a little delay, but like you said screensaver doesn't. I don't know if the delay could be due to some of my third party applications, because oddly enough I can start writing immediately and it takes in a few letters, but then stops and after like a second I can start writing again. So I'm thinking it could be an app stealing the focus for a second there...

I use Unlox and I noticed that when it locks my mac even with the screensaver locking, I can start writing just like with sleep. Unlox can lock the computer with or without the screensaver. The main point of unlox is that you can walk away from your computer with your iphone in your pocket and it gets locked when you get far enough (adjustable) and unlocks when you get close enough (adjustable). 

I guess I could also add that before I used unlox, I used to use Alfred to lock my computer manually, which has the system commands to put displays to sleep: sleepdisplays, or to put the whole computer to sleep: sleep and also screensaver, or to just lock the scren: lock. I think you can use these commands with the free version of Alfred.