A script that lets you press f when dragging a part to flip it. Only works on your own client.

flip.js

/*
 * A script that lets you press f when dragging a part to flip it.
 * Only works on your own client.
 *
 * by R26
 */
window.flipPartHooks = window.flipPartHooks || {
   design_onkeyup: designMode.onkeyup,
   design_rotatePart: designMode.rotatePart,
   design_draw: designMode.draw,
   part_draw: Part.prototype.draw,
};

flipPart = part => {
   if(part.dir === 0) part.dir = 4;
   part.dir *= -1;
   if(part.dir === 4) part.dir = 0;
};

designMode.onkeyup = function(e) {
   const part = this.draggingPart;
   if(e.key === 'f' && part?.flip) {
      flipPart(part);
   }
   flipPartHooks.design_onkeyup.call(this, e);
};

designMode.rotatePart = function(part, way) {
   const flip = part.dir < 0;
   if(flip) flipPart(part);
   flipPartHooks.design_rotatePart.call(this, part, way);
   if(flip) flipPart(part);
   return part
};

designMode.draw = function() {
   const part = this.draggingPart;

   if(part == null)
      return flipPartHooks.design_draw.call(this);

   const flip = part.dir < 0;
   part.forceFlip = flip;
   if(flip) flipPart(part);

   flipPartHooks.design_draw.call(this);

   if(flip) {
      if(this.draggingPart2?.flip)
         this.draggingPart2.dir = -((this.draggingPart2.dir + 3) % 4 + 1);
      flipPart(part);
   }
}

Part.prototype.draw = function() {
   if(this.forceFlip) flipPart(this);
   if(this.dir < 0) this.pos[0] *= -1;
   flipPartHooks.part_draw.call(this);
   if(this.dir < 0) this.pos[0] *= -1;
   if(this.forceFlip) flipPart(this);
};