In single window mode I experience a very annoying issue. One example to reproduce this: choose the brush tool, create a new layer and change it's opacity with the slider. And now it comes: I want to zoom into the image with the + and - keys on my keyboard. Or I want to move the displayed detail using the space bar - before I start painting...
In multi window mode one just had to focus the image window again (hit the window's title bar) to get those keys working. In single window mode, since all toolboxes belong to this one window, clicking the title bar doesn't change anything. Hitting + and - just changes the opacity of the layer, the space bar expands the selection box for layer modes or alters the layer's visibility. One actually has to paint onto the image to get it activated and get those keys working again. But when I want to paint, I need more details - a vicious circle...

Just now I help myself painting somewhere, hitting Ctrl-Z and finally being able to zoom in. But this is irritating. Maybe GIMP should check where the mouse pointer is and interpret the keystrokes based on this. Is the pointer on the image the user prob. wants to zoom, move etc. Is the pointer on the layer dialogue the user wants to change settings etc.

Actually, this is not an issue of the window modes. The same behaviour occurs in multi window mode. But, as described above, it's easier to re-focus the image window over there. Thus, this issue wasn't as annoying as it is now...
And it was always like this (at least as long as I remember).

The mailing list proposed to propagate every key stroke to the image window. But this would e.g. complicate detailed settings of sliders in the toolboxes using the left and right keys. Thus, I suggest the where-is-the-mouse-pointer query is a better idea...