Changing transparency of a whole Node2D with overlapping textured children

[DeRobyJ]

(TL:DR; pictures at the bottom)

Describe the project you are working on:
2D top-down videogame, undertale style

Describe the problem or limitation you are having in your project:
I need to change the transparency of "Room" nodes, which contain 3 tilemap nodes with overlapping tiles. Right now, the the "Room" parent node transfers its modulate to the children, thus changing its modulate.a property won't result in a homogeneous transparency, since overlapping sprites will have a different transparency than non-overlapping ones, because they are separate layer with that alpha value.

Describe how this feature / enhancement will help you overcome this problem or limitation:
This feature would allow me to change the overall transparency of the "Room", like if it were a ViewportTexture. I'm not using ViewportTexture because it gets overly complicated for just a graphical change. My character will have to interact to tiles based on their position, and using Viewports might affect existing game logic.

Show a mock up screenshots/video or a flow diagram explaining how your proposal will work:

Describe implementation detail for your proposal (in code), if possible:
Not applicable

If this enhancement will not be used often, can it be worked around with a few lines of script?:
Workarounds require the use of viewports, and it's not possible to work around the problem with shaders. You can find a discussion regarding this problem here: https://www.reddit.com/r/godot/comments/ertdgm/2d_transparency_of_overlapping_nodes_how_to/

Is there a reason why this should be core and not an add-on in the asset library?:
I believe this behavior is the expected behavior from the user's point of view. When changing the transparency of a node, we expect the whole node to become transparent, not its children.

For instance, open GIMP and do the following:

• Create an image
• Fill the background layer with black
• Create two layers with alpha channel turned on
• Draw two objects of different color, one per layer, and make sure they overlap
• Create a layer group
• Put the two layers with the drawings in the group
• Select the group
• Change transparency

This is the expected behavior.

Opacity 100% on all layers:

Opacity 50% on Layer Group, expected behavior:

Emulating Godot modulate.a behavior:

[Calinou]

This was implemented by godotengine/godot#43167, closing.