You need to enable the preview mode:

Open the command palette (Ctrl+Shift+P or Cmd+Shift+P on Mac)

Type: Preferences: Open Settings (UI) and then hit Enter

In the settings search bar, type: workbench.editor.enablePreview

Make sure the "Workbench › Editor: Enable Preview" is checked.

Enable the preview from Quick Open (it is optional):

Still in settings, check if "Workbench › Editor: Enable Preview From Quick Open" is also enabled.

This will make sure the single-clicks from things like Go to Symbol or Quick Open also use preview.