Godot 4 simple canvas item shader to add inner border.

inner_border.shader

shader_type canvas_item;

uniform vec4 border_color: source_color = vec4(1.0);
uniform float border_width: hint_range(0, 10) = 0;

void fragment() {
	float border_width_uv = border_width * max(TEXTURE_PIXEL_SIZE.x, TEXTURE_PIXEL_SIZE.y);
	
	if (step(0.5 - border_width_uv, abs(UV.x - 0.5)) == 1.0 ||
		step(0.5 - border_width_uv, abs(UV.y - 0.5)) == 1.0) {
		COLOR = border_color;
	}
}