Go to the Spot Removal tool when an image is presented at "FIT" magnification (although it can be reproduced in a number of scenarios, but this one is as good as any).
Enable Visualize Spots.
Go to 1:1 magnification.
Get out of Spot Removal.
While still at 1:1 magnification, if you move around (default hand tool) in the image, the "Visualize Spots" overlay is still active until you release the mouse button, after which it quickly disappears. This goes on until you perform another action, like going back to "FIT" magnification or moving a slider, but not when selecting a number of other tools, like Red Eye Correction, Graduated Filter, etc.