Visual Studio Code Review Difference Window

There should be a button on the toolbar like this one that will let you select the 4 different "diff modes".

You can also use the following keyboard shortcuts:

• Ctrl+\, Ctrl 1 - Inline
• Ctrl+\, Ctrl 2 - Side-by-side
• Ctrl+\, Ctrl 3 - Left Only
• Ctrl+\, Ctrl 4 - Right Only

to change the view.

Below screenshot will help in setting the option to compare side by side.