PCem

If I activate the "Remember size & position" option the machine dialog always appears in the same position, but the main window appears a few pixels below in the y axis from the last position. Every time I start a VM I have to move the window.

I haven't seen any other message about this, so, is there any other option I have to activate?

That might depend on the operating system.
I have had to fix this behaviour in other programs, when the window was opening not just a few pixels, but more than 20 pixels above the expected position, and the toolbar could move outside of the screen.
Also, on operating systems that use the X window (linux mostly), the window dimensions are not really available to the machine, since the window manager is generally responsible of adding that on top of the window. So this opens another can of try-and-error.

Here, on Windows 10, the configuration selector is always centered on screen, and the emulated screen only sometimes positions itself in a slightly lower position. But if i close it and open it again, it might open at the correct position, so it is not an incremental movement.

Every time I open the VM, the main window appears below the last position with the increase of some pixels. The machine window always opens in the same previous position.
If I don't move the window and keep opening and closing the program, the window goes off screen.

I am using Windows 7 x64 with a desktop resolution of 1280x720 with option Remember size & position activated.

Is this a Windows 7 issue only? Does not happen in Windows 10?

Now I've tested it with the released version of PCem (instead of my own compile) and indeed, it does what you describe.
My build is with msys2/gcc 7.3.0 and wxwidgets 3.0 while the compiled one is with mingw/gcc 4.9.2 and wxwidgets 3.1.

Have someone experienced this issue or depends on a very specific configuration?
Could be fixed for v14?