Easy way to choose whether to start in window or full-screen dynamically.

[HubKing]

Describe the project you are working on:
A beginner example.

Describe the problem or limitation you are having in your project:
Starting the game in a small window is usually better to quickly test something, but sometimes one would need to see it in the full screen (to see fine details, or in some cases, there are behavioural differences in window-mode and full-screen mode. Changing it in the settings (Run -> Window Placement) each time is not that convenient.

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
Any way to choose window/full-screen dynamically at the moment when the user invokes Play in the Editor, like:
*Method 1: An action for "Play in full-screen" on the Shortcuts dialogue. If that exists, one could map a key like Ctrl+F5 to it, and start the game in full-screen with that shortcut.
*Method 2: Clicking the Play button starts it in window, a modifier + click (like ctrl+click) starts it in full-screen.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
If the person want to start it in a window, he will click the play button or press F5. If he wants to start it in full-screen, he will ctrl+click the play button or press Ctrl+F5.

If this enhancement will not be used often, can it be worked around with a few lines of script?:
I would use this often. It should possible to programme a feature to toggle the full-screen mode in the game itself (seems to be no such shortcut exists by default, because pressing Alt+Enter did not do it), but then one would have to do that every time when they create a new project, and one would have to toggle the mode after the game has started.

Is there a reason why this should be core and not an add-on in the asset library?:
It is the editor's behaviour, so it probably is not possible with an add-on.

[Calinou]

It should possible to programme a feature to toggle the full-screen mode in the game itself (seems to be no such shortcut exists by default, because pressing Alt+Enter did not do it)

Since this is a widely used convention that's unlikely to conflict with existing keys in a game, we could have a default action for this (as long as the window is set to be resizable). Many popular games support this (even AAAs like GTA V).