As you can see in the effective ppi column in my links panel below, the selected image got slightly squeezed out of proportion by mistake. Probably by letting loose a tad too early of the Shift key.
Do you know of an easy way to reset the image proportions?
I know you could do something like "fit content proportionally", from the fitting options. However, that way the image gets completely repositioned within its frame, which is not very efficient in most cases.
And as you have probably already found, the little chain icon will constrain proportions, which means that you only have to enter either the X or the Y, and the other will replicate that number as well.
I often find it's easy to just hit one of the nudge buttons to change the scale factor to the next whole number, and both fileds will change to that. This does affect the fit, though, but usually not enough to be noticeable if the image is being cropped. You don't want to reduce the size if it will leave empty sapce inside the frame unless you subsequently fit frame to content.