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The Constrain To Image option doesn't lock the aspect ratio. For that, click the padlock icon:
The Constrain To Image option is the same checkbox as that labeled Constrain Crop in the Transform panel, which can create white bars on the sides of images:
The Constrain Crop / Constrain To Image option limits the crop to be wholly within the image, not including any of the white/grey bars:
That's confusing... When checked, a check box should do what it indicates.
This check box DOES do what it indicates:
it ensures that the crop boundary is Constrained to (cannot stray outside of) being entirely filled from the source image, as that is currently being transformed. For example, as you rotate an image's horizon.
What the checkbox does NOT do, is lock the current aspect ratio of the crop.
Which: it doesn't indicate that it will.
Well, it should lock the current aspect ratio of the crop. Still confusing.
In changing the crop's size and position (as and when needed, due to straying outside of the image area) turning on this Constrain option doesn't alter the crop's aspect ratio.
That is REGARDLESS of the state of the aspect 'padlock'.
So what are you really asking for here?
Obviously, a better designed feature.
It’s designed and named properly. In your earlier reply you said Constrain to Image should “lock the current aspect ratio of the crop,” but if so, then the change you propose would be confusing. Here is why: The existing name clearly says “Constrain to Image,” but you just said “lock the current aspect ratio of the crop.” Those are two different things.
For example, what if the image’s original aspect ratio is 3:2, but the current crop is 4:3 because it was changed? By your reasoning it would “lock the current aspect ratio of the crop” (4:3) not the of the image (3:2).
If you want to “lock the current aspect ratio of the crop” you should name your feature “Constrain to Crop,” not “Constrain to Image.” But as johnrellis already correctly showed, the option to “lock the current aspect ratio of the crop” already exists: It’s that existing padlock icon as shown. Notice that the lock icon is next to the aspect ratio menu; that’s how you know it locks the current aspect ratio.
“Constrain to Image” does what it says: It constrains the crop rectangle within the image borders.