In MWM, the image shifts inside the image window, plus some other problems
In Multi Window Mode, if the image window is sufficiently small:
When zooming in to see a particular portion of the image, the image will shift within the image window depending on whether the mouse is over or not over the image window.
When using the color picker, sometimes, maybe always (my test image was composed of large blocks of color which made it not possible to detect small shifts), and possibly depending on the desktop, this shifting of the image within the image window causes the picked color to be picked from a completely different spot than the spot from which the color was actually picked.
If the scrollbars are enabled, the vertical scrollbar disappears.
The behavior isn't dependent on the scrollbars. It still happens if the scrollbars are not visible and the navigator window is used for navigation within the image window.
I attached a video to show the behavior:
In the above video, the only color block that I picked colors from was the magenta block in the lower right corner. But sometimes the resulting color displayed in the Color Picker Information window is magenta, sometimes off-white, and sometimes green.
Another problem is shown in the above video, which is that in MWM, the Color Picker Information box has lowered contrast between the text and the background, as soon as the image window is too small to have the "on image" display. This is also true of the toolbox and all other dockers: unless the cursor is hovering over a given window, that window has decreased text/background contrast. This doesn't happen in SWM, and it does make
Sometimes, seemingly randomly - leastways I haven't figured out what triggers the behavior - it's impossible to actually make the image window any smaller, as shown by the video below:
When this "can't make the window smaller" problem happens, it seems the only way to be able to make the window smaller is to close and reopen GIMP.
One additional issue, for which I don't have a screen recording yet: Sometimes when picking colors from an image, if the Color Picker Information box is not "on the image" but is detached, then the Color Picker Information box disappears when the cursor is over the image, and only reappears when the cursor is moved off the image. If I manage to trigger this behavior again (it seems to happen fairly often), I'll make a screen recording.
I made the videos using Gentoo and IceWM, using GIMP's System theme. The system theme on my Gentoo install is Adwaita (there aren't any additional themes installed other than @bentou 's themes for GIMP).
To make sure nothing I normally have in my files for starting X and for modifying GTK3 behavior was triggering the image shifts and such, I commented out the lines that I had added to gtk.css and settings.ini in the folder $HOME/.config/gtk-3.0 and also commented out the GTK-related lines in my .xinitrc and .bashrc files, and then logged out and restarted X. But doing this didn't make any difference at all.
The same image-shifting behavior happens on OpenSUSE using various desktops. The GNOME and XFCE4 desktops on OpenSUSE probably don't have the "color picks from wrong area in the image" problem. I'm not sure about the text/background contrast issue on these desktops, but when I have a chance I'll double-check to be sure.