P: Photoshop 26.6.0 is selecting non-current canvases

I can confirm the same, this happens without the use of space bar, and on both over lapping and spaced out windows. But when using the space it rapidly flicks between the two windows. 

@CJButler  Ok I will test all of them. But just to quickly answer your theory. I'm afraid that's wrong. I don't need to use Space at all. It happens when I only hold down ctrl to auto-select a layer. Or when I want to pick a color using alt+ clicking mouse on the canvas. And it even happens without the canvases overlapping. I can have one canvas on the left side of the screen and try to select a layer on the right canvas, and suddenly the left/opposite canvas gets highlighted and the layers tab updaes with everything that the other image has and back again. So it flip flops and creates immense lags when you paint and pick colors constantly or select layers. If you understand what I mean?

@donbarrum 

Yes please, if you wouldn't mind. Try each of the suggestions one at a time. That would be helpful data.

I have a theory that might be in play: As you hit the space bar, and then drag with your mouse-down, is the space bar going up and down during the drag? Or is it just staying down? What I think is happening is that some other user interface element (I don't know which one - it is probably not be the other document window) is getting told to update itself, and it does a WM_ACTIVATE right in the middle of the drag. At that moment, the space bar is still down, so we get a WM_KEY event that gets redirected to the other (unknown) Window, which then decides to activate the Document Window that is in the background. I know a bug in this area was fixed this Friday, May 30, in our code base that will be released in 26.8, but I don't know if that fix affects the problem you are observing.

In general, if the update code for the unknown window decides that it doesn't need to do the WM_ACTIVATE, then the bug does not reproduce, which is why it is not seen by everyone. Furthermore, this behavior is all Windows only. Mac OS users would not be affected. /EndTheory

That "Modern UI" switch helps localize the code that possible caused the issue. Eventually this will be the new code as we retrofit Photoshop to be more modern. But that's a long-term task, and will likely take over a year. It's akin to changing parts on a car while everyone is still driving it. I won't comment on the strategy, as that decision has been made.

Hi, I just tried. Good news, I guess, the first thing you suggested, turning off the "Enable Modern User Interface" fixed it right away.  I don't know what modern user interface does to the UI ? But at least that is the culprit. Do you want me to test the other things on and off as well?

@donbarrum 

We're still trying to reproduce. A couple of things to try that may help narrow things down:

1. Try turning OFF Edit > Preferences > Technology Previews... > Enable Modern User Interface
2. Try turning OFF Window > Contextual Task Bar
3. What happens when you don't use Floating Windows, but instead use Docked Windows in the Application Frame. In that case, the foreground window should cover the entire canvas space. Do you still get unexpected switching to a different (background) window?

And some questions:

1. Is Edit > Preferences > Tools > Overscroll ON or OFF?
2. Do you normally run with Move Tool > Auto-Select = OFF, and then use the space bar (a keyboard event) to switch to the Hand Tool? Is that your normal workflow? What happens if you switch to the Hand Tool directly, without using the space bar to switch from the Move Tool to the Hand Tool?

Let me know if there's anything I can assist with, shared screen session or whatever. 

Nothing is ruled out at this point, although I do have and use a Wacom Cintique on one of my Windows System. That was the first one I tried. It is also a dual monitor setup. I watched the videos and tried for a bit, but could not reproduce. We'll keep looking and see if we can find someone internally who has an environment that can reproduce. Right now, it's a mystery.

This problem is still persisting for me to, it's becoming quite annoying now. I have photoshop updated to the latest version, basic system spec as follows:

Windows 11 pro (all updates installed) 

AMD 9950x

96GB Ram

Geforce RTX 5080 on Studio drivers (latest update installed)

Operating dual 1440 monitors

thanks for investigating. That's very strange you cannot reproduce it, since I have it on both my computers and they have different cpus and hardware. One is amd and the other is intel. Only thing in common is the Nvidia card. But could it be because I have photoshop on a second display, a wacom display? There's quite a bit of bugs with photoshop that appear with a wacom that you don't experience when using a mouse and one screen. 

We're looking at it now. I've tried with a couple of versions of PS but cannot reproduce. I've asked another engineer to help, but so far we cannot, so I expect there's some secret sauce we're missing. The bug is still open and I agree it's annoying. We'll keep trying.