shader_type canvas_item;
uniform sampler2D mask; // a texture
uniform float progress : hint_range(0.0, 1.0, 0.001);

void fragment() {
    float mask_value = texture(mask, UV).r;

    // ensure the progress is a bit bigger than the feather
    float threshold = progress * 1.001;

    // Fade alpha based on mask vs. threshold with a feather
    COLOR.a *= smoothstep(threshold, threshold * 0.999, mask_value);
}