allow z middle anchor offsetting, mostly driven by lathe requirements

src/kiri/platform.js

@@ -147,7 +147,7 @@ function platformUpdateMidZ() {
     space.platform.setMaxZ(topZ);
 }
 
-function platformUpdateTopZ(zdelta) {
+function platformUpdateTopZ() {
     const { process, stock } = current();
     const MODE = get_mode();
     api.widgets.each(widget => {
@@ -160,7 +160,11 @@ function platformUpdateTopZ(zdelta) {
                    widget.setTopZ(stock.z - zdelta);
                    break;
                case 'middle':
-                   widget.setTopZ(stock.z - (stock.z - wzmax) / 2);
+                   if (widget.isIndexed) {
+                       widget.setTopZ(zdelta);
+                   } else {
+                       widget.setTopZ(stock.z - (stock.z - wzmax) / 2 + zdelta);
+                   }
                    break;
                case 'bottom':
                    widget.setTopZ(wzmax + zdelta);

src/kiri/widget.js

@@ -430,6 +430,10 @@ class Widget {
         }
     }
 
+    get isIndexed() {
+        return this.track.indexed ? true : false;
+    }
+
     setIndexed(z) {
         if (z !== this.track.indexed) {
             this.track.indexed = z;
@@ -456,7 +460,7 @@ class Widget {
             track = this.track,
             mbb = mesh.getBoundingBox(),
             mbz = mbb.max.z;
-        if (z) {
+        if (z != undefined) {
             track.top = z;
         } else {
             track.top = mbz;
@@ -510,7 +514,7 @@ class Widget {
             let dx = Math.sin(rad) * ln;
             let dy = ln - Math.cos(rad) * ln;
             this.mesh.rotation.x = -rad;
-            z = 0;
+            z = top;
         } else {
             this.mesh.rotation.x = 0;
             z += tz;