Dock keeps moving to second monitor

This is expected behavior.

When you have two monitors side by side, and the Dock is positioned at the bottom, it will appear on either display, when you move the cursor to the bottom of that display.

If you want the Dock to only appear on one display, you have two options:

a) Uncheck "Displays have separate Spaces"

or

b) Place the Dock on one side instead of the bottom

Personally I would not want to use option a), but since you also seem to like having a window span across more than one screen, this is the option that will give you both things.

Of course, this means you lose some other things - no more having the menu bar on both displays; fullscreen applications basically will leave the second screen black and unusable... not my cup of tea, but maybe it will work for you.

Note: a change to this setting will only take effect after restarting

The dock will attach itself to the monitor of the menubar, except when it is pushed to the side that is being extended, in which case it pushes itself to the edge of the extension.

Apple menu -> System Preferences -> Displays lets you in the arrangement lets you drag the menubar from one display to another as it is represented in the arrangement field. And the extension is manages by how the monitor representations there are aligned to each other.

Currently it does not work the way you want.

Having menu bar on both displays requires “separate Spaces”, and that in fact means what it says. The downside is that a window can’t be in both at the same time.

Maybe there is a third part system modification that makes the Dock show on all displays. I honestly think that is a waste of screen real estate but I would not mind if there were a preference to turn that on.

That is something only Apple can implement so use feedback and make a request.

http://www.apple.com/feedback

I am puzzled by your "I need all the screen I can get" claim! Ok, I understand you want that, then your options about the Dock do not match.

I would expect you'd want the Dock to be as much out of the way as possible. So:

Where is that? It should be obvious:

a) Keep the Dock hidden - so your document windows can use all the screen real estate and not keep bumping into the Dock.

b) With two screens, side by side, you screen area is more than THREE times as wide as it is tall. (With three, it would be more than 4 and a half, possibly five times as wide!)

Where is the Dock more out of the way? On the side, of course.

On a side note: I am continuously surprised with how much people love or hate the Dock. I can work for a whole day and never touch it or even see it (as you may have guessed, it is hidden all the time on my mac).

if one hold the cursor on the other displays lower edge after x sec the dock will be moved to that monitor, not sure there is a way to disable this behavior

fullscreen applications basically will leave the second screen black and unusable...

What do you mean?

Have you tried putting an application in full screen when you have the menu bar only on the main screen (i.e. "separate Spaces" is OFF)?

Granted, it has been years since I did that, because I require "separate Spaces", and never turned it off (since Mavericks?) but that is what used to happen. Application using the main screen in full, secondary screen turned black. Did that change?

"Having the dock at the bottom is fine, I need something to switch between/open apps after all, somewhere that's easy to reach, so either on the main screen or both on my main screen and on my second screen."

Why switch apps using the Dock?

Don't you know about Command-Tab?

No need to take your hands off the keyboard.

Simply pressing Command-Tab switches to most recent used application.

Or press command-tab, keep holding the command key and use the arrow keys to switch to another application.

Command-` switches between windows of the same application.

There is also Mission Control.

You can summon Mission Control, for example, with control-up arrow, and have access to all windows in all applications.

You can also summon all windows from just one application with control-down arrow.

Single-window applications - like, for example, System Preferences - quit when their only window is closed.

That makes sense for those applications, because all there is to do with them is in that window.

Multiple-window application - like, say, Safari - don't. They can have 1,2,3... 10, 100 windows open... or zero.

That is how a mac has always behaved.

Many people that are accustomed to Windows think that clicking the red button closes the application.

That is not how it works in the mac. To quit, press command-Q or choose Quit.

(And, incidentally, only rarely do I click the red button, anyway; command-W closes tabs or windows; command-option-W closes all windows; but still does not quit an application)

macOS is different from Windows.

[In fairness to Windows: the command-tab is one of the few things that started in Windows first - there, it's alt-tab, but it has the same functionality]