Unexpected rendering with screen reading shader, Light2D and LightOccluder2D

[djrain]

Godot version:
3.3 RC7

OS/device including version:
2020 Mac Mini
macOS Big Sur
GLES2 / GLES3

Issue description:
I have a 2D water shader which reads from SCREEN_TEXTURE to display whatever is behind with a refraction effect. A ColorRect and a Sprite are placed behind the water, and a Light2D affects everything. There is also a LightOccluder2D covering the sprite, set to clockwise cull mode so that the inside of the sprite remains lit.

The issue is that in the water, this results in two visible copies of the screen texture being lit, one (correctly) distorted by the shader, and the other not:

Whereas I would hope for it to look more like this:

Water shader:

shader_type canvas_item;

void fragment() {
	vec2 coord = SCREEN_UV + vec2(sin(TIME*3.0 + UV.y*10.0) * 0.02, 0.0);
	vec4 screen = textureLod(SCREEN_TEXTURE, coord, 0.0);
	COLOR = mix(vec4(0.2, 0.6, 1.0, 1.0), screen, 0.5);
	COLOR.a = 1.0;
}

The result is the same whether batching is on or off.

Steps to reproduce:
Simply run the MRP scene.

Minimal reproduction project:
ScreenTextureLightIssue.zip

[clayjohn]

Have you tried using a backbuffercopy? I wonder if the shadows are drawn after screen_texture is captured.

[djrain]

Have you tried using a backbuffercopy? I wonder if the shadows are drawn after screen_texture is captured.

Yeah, I tried it in various places and it didn't help.

[Calinou]

Related to #44066.