Show Scene camera preview in Game editor when not running

[kitbdev]

Describe the project you are working on

The Godot editor

Describe the problem or limitation you are having in your project

The Game editor is mostly empty when a game is not running in it. This space can be used better.

• Related Add "Game" editor for better runtime debugging godot#97257 (comment)
• Related Add picture-in-picture camera previews to the editor when a Camera3D node is selected #1017

Describe the feature / enhancement and how it helps to overcome the problem or limitation

A game camera preview can be shown here at all times that the game is not running in it. Unlike the preview when clicking on a Camera3D, this would be a 2D and 3D preview that is available even when the camera is not selected.
The goal is to make a preview of what would show when the scene is run.
This way the Game tab is useful even when not running a game.
Also this is what Unity does with its Game view.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

The Game Editor tab would show a live preview of the selected scene, as an estimate of what the game shows when run.
It should take into account the target resolution as well.

The current text that shows the status would either be moved to the bottom, or superimposed on top of the preview, to make it clear that it is only a preview and not interactable.

I imagine the implementation would be similar to the Camera3D preview godotengine/godot#90778, but it finds the current camera and handles 2D rendering as well.

If this enhancement will not be used often, can it be worked around with a few lines of script?

No.

Is there a reason why this should be core and not an add-on in the asset library?

To improve the default behavior of the editor and fill unused space.

[SilverWolveGames]

As an ex-Unity user, this feature would be a very great addition to Godot.

[Calinou]

This may be confusing if you're not aware that the project is not currently running though. You may wonder why your game code isn't running even though you're seeing a 2D/3D view in the Game tab.

[Elip100]

This may be confusing if you're not aware that the project is not currently running though. You may wonder why your game code isn't running even though you're seeing a 2D/3D view in the Game tab.

There could be an indicator stating that the game is not running