A simple button snippet for 1.8.9 Sponge mixins. (Original by Tascord, Mixin port by me)

MixinGuiButton.java

// | ---------------------------------------------------------------------- |
// | Created by Nebula (http://bit.ly/subnebula)                            |
// | This snippet is for you to LEARN from. NOT copy.                       |
// |                                                                        |
// | This program is free software: you can redistribute it and/or modify   |
// | it under the terms of the GNU General Public License as published by   |
// | the Free Software Foundation, either version 3 of the License, or      |
// | (at your option) any later version.                                    |
// |                                                                        |
// | This program is distributed in the hope that it will be useful,        |
// | but WITHOUT ANY WARRANTY; without even the implied warranty of         |
// | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the          |
// | GNU General Public License for more details.                           |
// |                                                                        |
// | You should have received a copy of the GNU General Public License      |
// | along with this program.  If not, see <https://www.gnu.org/licenses/>. |
// |                                                                        |
// | Installation steps:                                                    |
// | 1. Download mod into src/java/clientname/mixins/client/gui             |
// | 2. Add the file to your mixins json                                    |
// | 3. Enjoy!                                                              |
// | ---------------------------------------------------------------------- |

// You will need to change the package name here
package xyz.nebulayt.cobalt.mixins.client.gui;

import net.minecraft.client.Minecraft;
import net.minecraft.client.gui.FontRenderer;
import net.minecraft.client.gui.Gui;
import net.minecraft.client.gui.GuiButton;
import net.minecraft.client.renderer.GlStateManager;
import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.Shadow;

import static net.minecraft.client.gui.Gui.drawRect;

@Mixin(GuiButton.class)
/** Mixin for GuiButton
 * @author Nebula | Nebula#9998
 * @since 1.0.0 **/
public class MixinGuiButton {

    @Shadow protected int width;
    @Shadow protected int height;

    @Shadow public int xPosition;
    @Shadow public int yPosition;

    @Shadow public boolean visible;
    @Shadow protected boolean hovered;

    @Shadow public boolean enabled;

    @Shadow public String displayString;

    @Shadow protected int getHoverState(boolean mouseOver) {
        int i = 1;

        if (!this.enabled)
        {
            i = 0;
        }
        else if (mouseOver)
        {
            i = 2;
        }

        return i;
    }

    @Shadow protected void mouseDragged(Minecraft mc, int mouseX, int mouseY) { }

    protected void drawHorizontalLine(int startX, int endX, int y, int color)
    {
        if (endX < startX)
        {
            int i = startX;
            startX = endX;
            endX = i;
        }

        drawRect(startX, y, endX + 1, y + 1, color);
    }

    protected void drawVerticalLine(int x, int startY, int endY, int color)
    {
        if (endY < startY)
        {
            int i = startY;
            startY = endY;
            endY = i;
        }

        drawRect(x, startY + 1, x + 1, endY, color);
    }

    public void drawCenteredString(FontRenderer fontRendererIn, String text, int x, int y, int color)
    {
        fontRendererIn.drawStringWithShadow(text, (float)(x - fontRendererIn.getStringWidth(text) / 2), (float)y, color);
    }

    public void drawButton(Minecraft mc, int mouseX, int mouseY) {
        if(visible)
        {
            FontRenderer fontrenderer = mc.fontRendererObj;

            drawRect(this.xPosition, this.yPosition, this.xPosition + this.width, this.yPosition + this.height, 0x44000000);

            int outlineColour = 0x44555555;
            this.drawHorizontalLine(this.xPosition, this.xPosition + this.width, this.yPosition, outlineColour);
            this.drawHorizontalLine(this.xPosition, this.xPosition + this.width, this.yPosition + this.height, outlineColour);
            this.drawVerticalLine(this.xPosition, this.yPosition + this.height, this.yPosition, outlineColour);
            this.drawVerticalLine(this.xPosition + this.width, this.yPosition + this.height, this.yPosition, outlineColour);
            //           this.drawVerticalLine(x+w, y+h, y, colour);

            GlStateManager.color(1.0F, 1.0F, 1.0F, 1.0F);

            this.hovered = mouseX >= this.xPosition && mouseY >= this.yPosition && mouseX < this.xPosition + this.width && mouseY < this.yPosition + this.height;
            int i = this.getHoverState(this.hovered);

            GlStateManager.enableBlend();
            GlStateManager.tryBlendFuncSeparate(770, 771, 1, 0);
            GlStateManager.blendFunc(770, 771);


            this.mouseDragged(mc, mouseX, mouseY);
            int j = 14737632;

            if (!this.enabled)
            {
                j = 10526880;
            }
            else if (this.hovered)
            {
                j = 16777120;
            }

            this.drawCenteredString(fontrenderer, this.displayString, this.xPosition + this.width / 2, this.yPosition + (this.height - 8) / 2, j);
        }
    }

}