How to LOCK the dock on the LEFT on the macbook main screen?

You can use the feedback page to make a request:

https://www.apple.com/feedback

AFAICT, currently there is no way to force the Dock to be placed the way you describe.

I understand your point about seeing the red dots; that seems a good reason to keep the Dock on. To switch to any application, I suggest you use Command-Tab, which is far quicker than moving the cursor to the Dock to click.

Try this System Settings > Desktop & Dock > Mission Control > turn on Displays have separate Spaces

Mal-S wrote:

Try this System Settings > Desktop & Dock > Mission Control > turn on Displays have separate Spaces

That should certainly do what the OP asked, but:

• there will only one menu bar in the center screen;
• if one puts an application in Full Screen the two side screens will black out...

I am curious to learn how important this is worth the effort for the OP and whether these compromises are worth making just for putting the Dock in that position.

And now, finally, I had some time to actually test...

Here are my findings:

1) Even with "Displays have separate Spaces" turned OFF, the Dock will still be on the extreme left (or right), so no way to put it in the left side of the middle display.

2) It is still the case, as I expected, that with the setting OFF, putting an application in Full Screen turns the secondary display(s) black and uses only one display.

I always have “displays have separate Spaces” turned ON too.

My comment about other screens being blacked out, and about the Dock being in the left side of the center screen, applied to the situation were “displays have separate Spaces” is OFF.

I don’t advise to have it off, but I think it’s the only way for the OP to have the Dock where he wants it.

To be clear, you have “Displays have separate Spaces” off, right?

That’s interesting. I will have to experiment. I admit that I have only turned this setting off once or twice, for testing, ever since Mac OS X Mavericks; it is quite possible that if has changed at some point and I may have missed it.

Quote.

That should certainly do what the OP asked, but:

• there will only one menu bar in the center screen;
• if one puts an application in Full Screen the two side screens will black out...

Unquote

I have this setting with a single external display attached to my M1 iMac and both screens act independently, full screen does not blackout the attached second display

No. When positioned on one side it will go to that side. Otherwise it would be in the middle. Why would you want to go through the Dock when moving from one screen to the other?

Luis,

Displays have separate Spaces is turned on for my set up