There's a very annoying sizing bug in Lightroom for Mac
* open a photo into full-size view (ie double-click it in grid view)
* two-finger-pinch to zoom it in and out.
* click Fit to resize it to fit
=> the image view will be askew and not aligned with the actual frame of Lightroom anymore. this will persist as you switch between images with cursor left/right, or even if you go back to the grid view and then open another picture.
=> you can click between 100% and Fit, and every time, Fit will be bad
* now drag and resize the Lightroom window (or exit/re-enter Full-screen
=> the image will snap back into frame
This video shows this; i'll also attach two screenshots,