Here are a number of shortcuts to help navigate the before and after views in Lightroom’s Develop module:
• “\” toggles Before and After view full screen (one view showing at a time).
• “Y” displays Before and After (Left/Right) View. Add the Option (Mac) / Alt (Win) to toggle Before and After Top/Bottom view.
• Shift + Y toggles to Split Screen view.
• Drag and drop the name of any state from the History panel on top of the image in the “Before” preview area to compare with the current state (this can be infinitely more useful then comparing the original “Import” state!)
• Control (Mac), right mouse (Win) -click on a Snapshot to “Copy Snapshot Settings to Before” preview in Before/After view.
• Command + Option (Mac) / Control + Alt (Win) + Shift + Left/Right arrows move the After state to Before/Before state to After.
• Command + Option (Mac) / Control + Alt (Win) + Shift + Left arrow moves the After state to Before.
• Command + Option (Mac) / Control + Alt (Win) + Shift + Right arrow moves the Before state to After.
• Command + Option (Mac) / Control + Alt (Win) + Shift + Up arrow swap the Before and After state.
And, if you only want to preview the effects of a certain panel, click the toggle switch at the top of the panel to temporarily hide the panel’s effects (except the Basic panel – it does not have this switch).