How do I keep the Dock at the bottom of my main screen when using a secondary monitor?

The only way to solve it and not disable "Displays have separate Spaces" is with the app DockLock Lite from the Mac App Store.

It locks the Dock at the bottom of the selected screen as you want it to.

Hi,

Following methods could help your question.

To keep the Dock at the bottom of your main screen when using a secondary monitor, you need to configure Mission Control and System Settings. Uncheck "Displays have separate Spaces" in Mission Control to prevent the Dock from switching between monitors. Then, in System Settings > Desktop & Dock > Dock, ensure the "Position on screen" is set to "Bottom". 

Detailed Steps:

1. 1. Mission Control Settings:

• • Open System Settings (Apple menu > System Settings).
  • Click "Desktop & Dock" in the sidebar.
  • Go to the "Mission Control" section.
  • Uncheck the box next to "Displays have separate Spaces".
  • You may need to log out and log back in for the changes to take effect.

1. 2. Dock Position:

• • In the "Desktop & Dock" settings, locate the "Dock" section.
  • Click the "Position on screen" dropdown and select "Bottom".
  • This will ensure the Dock stays at the bottom of your main screen, even when using a secondary monitor. 

Explanation:

By unchecking "Displays have separate Spaces," you tell macOS to treat all your displays as a single desktop, so the Dock remains on the same screen regardless of which display you're actively using. Setting the Dock's position to "Bottom" ensures it's always at the bottom of your main screen. 

DockLock Lite solves the problem. I tried it and it works well.