How to integrate relative mouse movement while the mouse is captured in a HTML5 game?

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:bust_in_silhouette: Asked By fviktor

Environment: Windows 10 Pro 64bit, Chrome, Godot 3.0.2, GDScript

Capturing the mouse must be done in an input handler like _input on HTML5 due to browser security restrictions. (See
What is the proper way of capturing the mouse in a HTML5 game? - Archive - Godot Forum for more details.)

While the mouse is captured the following code is used to integrate (aggregate) the relative mouse movements:

func _input(event):
    if event is InputEventMouseMotion:
        aggregated_mouse_delta += event.relative

It works fine on desktop, but on HTML5 event.relative seems to be zero all the time.

Where the relative movement vector is saved while the mouse is captured in a HTML5 game?