There are several options for when the "Combat Viewer" will appear. These are in the Options menu, in the "Interface" section.
If you would like to see the combat viewer for any battle, use the "Full Battles" option and also check "Full Fleet Battles" and uncheck "Show Full Fleet Battles Only When Both Sides are Fleets"
If you would like to just see the battles for any battle involving a fleet, change your Battle type to "Normal Battles", check "Full Fleet Battles", and uncheck "Show Full Fleet Battles Only When Both Sides are Fleets"
If you would like to see the battles only when a fleet is engaging another fleet, set battle type to "Normal Battles"," and check both "Full Fleet Battles" and "Show Full Fleet Battles Only When Both Sides are Fleets"
You can also try other combinations of these options to find one that suits you.