What is the thumbnail size?
She has the Thumbnail Size set to None
No, I don't. The Thumbnail size is set to small, as it has been forever.
Please refer to the image attached above.
Then I guess your image is too broad to display the »detailled« icons in the Layers Panel.
No, the image is not too broad - it is a 27-inch screen with lots of space around the image.
You don’t seem to understand, your screen is not the relevant factor.
Set the thumbnails to a bigger size already …
And as concerns the influence of the width of an image on the Layers Panel’s display at small thumbnail size:
Thank you for your response, but it does not fix my problem.
Did you try a bigger thumbnail size?
Did you try a less wide image dimension? (This would naturally not address the issue itself but just illustrate the principle how canvas dimensions influence the height of the icons on the Layers Panel.)