ScrollBoxMethod.java

@Override
public void drawForeground(Point mouse) {

    // GuiUtils.drawRectangle(contentArea, Color.MAGENTA);

    Point drawOffset = displayedContentArea.getLocation();
    drawOffset.translate(contentArea.x, contentArea.y);

    GlStateManager.pushMatrix();
    GlStateManager.disableLighting();

    StencilBuilder db = new StencilBuilder()
        .unmask((VertexBuffer vb) -> {
            vb.begin(GL11.GL_QUADS, DefaultVertexFormats.POSITION);
            vb.pos(contentArea.x + contentArea.width, contentArea.y + contentArea.height, 0).endVertex();
            vb.pos(contentArea.x + contentArea.width, contentArea.y, 0).endVertex();
            vb.pos(contentArea.x, contentArea.y, 0).endVertex();
            vb.pos(contentArea.x, contentArea.y + contentArea.height, 0).endVertex();
        }).startDraw();

    GlStateManager.pushMatrix();
    elementDisplayCache.forEach(e -> e.drawForeground(mouse));

    GlStateManager.popMatrix();

    db.endDraw();
    GlStateManager.popMatrix();
}

StencilBuilder.java

package codebirb.guilib.utils;

import codebirb.guilib.utils.stencil.StencilFunctionOption;
import net.minecraft.client.renderer.Tessellator;
import net.minecraft.client.renderer.VertexBuffer;
import org.lwjgl.opengl.GL11;

import java.util.function.Consumer;

import static org.lwjgl.opengl.GL11.*;

public class StencilBuilder {

    private Consumer<VertexBuffer> stenciler;

    public StencilBuilder() { }

    public StencilBuilder unmask(Consumer<VertexBuffer> stenciler) {
        this.stenciler = stenciler;
        return this;
    }

    private void enable() {
        glEnable(GL_STENCIL_TEST);
    }

    private void disable() {
        glDisable(GL_STENCIL_TEST);
    }

    private void clear() {
        glClearStencil(0);
        glClear(GL_STENCIL_BUFFER_BIT);
    }

    @Deprecated
    public static void stencilOption(StencilFunctionOption opt, int val) {
        int STENCIL_TEST = glGetInteger(GL_STENCIL_FUNC);
        int STENCIL_FAIL = glGetInteger(GL_STENCIL_FAIL);
        int DEPTH_FAIL = glGetInteger(GL_STENCIL_PASS_DEPTH_FAIL);

        int glDefault = GL_KEEP;

        switch(opt) {

        }
    }

    private void startDrawingMask() {
        glStencilMask(0xFF);
    }

    private void finishMask() {
        glStencilMask(0x00);
    }

    public StencilBuilder startDraw() {
        enable();
        clear();
        // GL11.glClear(GL11.GL_STENCIL_BUFFER_BIT); // Flush old data

        startDrawingMask();
        // GL11.glStencilMask(0xFF); // Writing = ON

        GL11.glStencilFunc(GL11.GL_ALWAYS, 1, 0xFF); // Always "add" to frame
        GL11.glStencilOp(GL11.GL_KEEP, GL11.GL_KEEP, GL11.GL_REPLACE); // Replace on success

        Tessellator t = Tessellator.getInstance();
        VertexBuffer vb = t.getBuffer();
        this.stenciler.accept(vb);
        t.draw();
        finishMask();
        // GL11.glStencilMask(0x00); // Writing = OFF

        GL11.glStencilFunc(GL11.GL_NOTEQUAL, 0, 0xFF); // Anything that wasn't defined above will not be rendered.

        return this;
    }

    public void endDraw() {
        clear();
        disable();
    }
}