Thank you for your help. What follows is no doubt specific to my case, but perhaps it is still relevant:
1. I like to work with one screen directly in front of me, where I do most of my work, and a second auxiliary screen to one side, where I can arrange multiple figures, lists of data, etc.
2. Physical space constraints and habit place the second (auxiliary) screen to the actual left of the main, central screen.
3. The Windows task bar (with Start button, etc.) is displayed on the "primary" screen.
Because I'd like to have the main Windows UI components (i.e. task bar, new windows, dialogs) appear on my main, central screen, per (3) I designate the main, central screen as "primary" in the Windows display settings tool. Note that this does NOT mean that it is numbered/identified as "1". Any screen (here, "1" or "2") can be the "primary" screen.
Because I'd like to have my mouse movements and windows continue properly across screen boundaries, I make sure that Windows knows the proper relative positions of the screens ("primary" in the center, and another screen to the left of that).
For some reason (I don't know how this is determined), the "primary" screen is numbered "2", and the other screen, to the left, is numbered "1" in the Windows display settings.
As a result, the configuration looks like this: 1 where  denotes primary.
hines wrote:If you pop up the Print and file window manager, does the screen icon seem to reflect the
relative position of your screens?
Not correctly. The PFWM display does include the extra screen space, but it shows up to the right of the primary display. Any window that I drag to this area in PFWM, disappears "off-screen". Any window that I drag to the left additional screen is shown outside of the screen boundary in PFWM.
Isn't there a windows tools that shows the arrangement of your screens? If so does it show your screen 2 is on the
left or right of screen 1?
Screen 2 is to the right of screen 1. But screen 2 is the primary display.
I believe the problem will go away if your windows display tool is used to
put the second screen just to the right of your main screen. Is there some other reason that that would be inconvenient?
I hope my description above answers this. In short - yes. My auxiliary screen is to the left of my main, central screen. As far as the "numbering" is concerned, perhaps I can find some way to reassign "1" to the primary display, if that would help, but I'm not sure if/how that is possible, or helpful.
I see now that all dialogs (not just this Import 3D file browser) are affected in this way, so my workaround may be too cumbersome. Unless there is a fix for this issue (I believe Windows may use negative screen space coordinates for left-of-primary), it looks like I will have to either (a) detach the 2nd monitor when working with NEURON, or (b) find a way to change my physical configuration so that the primary screen is always on the left.