So, I'm using the single-window mode.

The issue: the space key (panning key) responds to image panning only after "selecting" the main image area in an "on-click" manner.

Here are the repro steps:
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1. Start up GIMP, create a new image (any image)
2. Hover over the main image area with your cursor, press and hold the space bar (panning key), and while still holding it, move the mouse/tablet around.
---> Panning works.
3. Use the cursor to activate some elements outside of the main image area (add another layer, for example).
4. Return to the main image area - hover over it with the cursor.
5. Press the space bar (panning key) again.
---> Panning does not work; instead, the space bar activates some element from the side area (i.e. shows/hides the newly created layer, etc.)
---> Panning should work here, after hovering the cursor over the main image area; currently, panning works only by attaining an on-click focus.

This is not a critical issue. But it is an annoying, counter-intuitive one.
Here are the reasons:
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A. It is counterproductive. For example, if you have a zoomed in image in the main area, a brush tool on your cursor and you find yourself fiddling with the layers and then returning to the main image area to pan the focused image elsewhere, you have to choose another, non-editing tool to get the focus on the main image area, because clicking on the main image area with a brush selected would leave an undesirable mark.
This process requires a few extra clicks, and some caution/awareness. It is just a few clicks, but try doing that one hundred times while painting something (futile!)

B. It is inconsistent. The other panning "key" (middle mouse button) pans every time you use it while hovering over the main image area. In a similar fashion, the space key should also work whenever you use it while hovering over the area.
"If you don't like the space bar behavior, then use the middle mouse button" is not a valid argument here, as some tablet/pen setups don't have that button in a particularly practical place to reach.

C. It is non-standard. I.e. the space bar behavior i proposed is  not just a product o my imagination - it is being used in Photoshop (and it does work quite well).