fix(zpl): reverse text uses ^GB+^FR knockout pattern instead of ^LR

^LRY/^LRN wraps a single field with label-reverse, which Zebra
firmware (and Labelary) treat as a global bitmap flip rather than
per-field inversion. The result: our reverse-text emit produced no
visible inversion on print, only the canvas faked the effect.

Switch the text emitter to the standard ZPL pattern for white-on-
black: a filled black ^GB at the field anchor (sized to the rendered
ink bbox) followed by ^FR^FD so the printer knocks the glyphs out of
the black.

Parser stashes filled-black ^GB candidates and collapses them with a
following ^FR text at the same anchor with matching bbox back into a
single reverse-text object, so the round-trip stays one-to-one. Hand-
written ZPL with unrelated ^GB+^FR pairs at mismatched anchors or
sizes round-trips unchanged via the existing box path.

Scope limited to text per the open follow-up ticket; box / ellipse /
line still emit ^LR via wrapReverse and remain broken on Labelary
(rarely used; addressed separately).

Author: u8array

src/components/Canvas/KonvaObject.tsx

@@ -277,8 +277,14 @@ function KonvaObjectInner({
       // Use the measured ink width (already in dot space, mirrored to
       // CSS px via dotsToPx) so the inverted background bbox tracks the
       // actual rendered text rather than a length-based guess.
+      // Match the printer's ^GB knockout-background exactly: width =
+      // measured ink width, height = font height in CSS px. Earlier
+      // versions padded the height to 1.3× for visual breathing room
+      // but that hid what the printer actually emits — the canvas now
+      // mirrors the same `^GB inkW, fontHeight` shape the generator
+      // produces, so the preview matches Labelary and the real device.
       const approxW = dotsToPx(textMetrics.inkWidthDots, scale, dpmm);
-      const approxH = fontSizePx * 1.3;
+      const approxH = fontSizePx;
       return (
         <Group
           id={obj.id}
@@ -309,7 +315,7 @@ function KonvaObjectInner({
             fontStyle="bold"
             scaleX={fontScaleX}
             fill="#ffffff"
-            y={approxH * 0.1}
+            y={0}
           />
         </Group>
       );

src/lib/zplGenerator.test.ts

@@ -55,6 +55,27 @@ describe('generateZPL — structure', () => {
     expect(zpl).toContain('^LS5');
   });
 
+  it('reverse text round-trips: emit ^GB+^FR, parse collapses back to one reverse text', () => {
+    // Generator emits a filled black ^GB knockout-background followed by
+    // an ^FR text at the same anchor. The parser detects that pair and
+    // collapses it back into a single text object with reverse:true —
+    // so the editor never sees two objects after a save/load cycle.
+    const objs = [
+      { id: 'r', type: 'text', x: 50, y: 50, rotation: 0,
+        props: { content: 'Hi', fontHeight: 30, fontWidth: 0, rotation: 'N', reverse: true } },
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    ] as any;
+    const zpl = generateZPL(BASE_LABEL, objs);
+    expect(zpl).toContain('^GB');
+    expect(zpl).toContain('^FR^FD');
+    expect(zpl).not.toContain('^LRY');
+    const { objects } = parseZPL(zpl, 8);
+    expect(objects).toHaveLength(1);
+    expect(objects[0]?.type).toBe('text');
+    expect(props(objects[0]).reverse).toBe(true);
+    expect(props(objects[0]).content).toBe('Hi');
+  });
+
   it('omits objects with includeInExport=false', () => {
     const objs = [
       { id: 'a', type: 'text', x: 10, y: 10, rotation: 0, props: { content: 'KEEP', fontHeight: 30, fontWidth: 0, rotation: 'N', reverse: false } },

src/lib/zplParser.test.ts

@@ -136,6 +136,19 @@ describe('parseZPL — ^FR field reverse', () => {
     const { objects } = parseZPL('^XA^FO0,0^A0N,30,0^FDNormal^FS^XZ', 8);
     expect(props(objects[0]).reverse).toBeFalsy();
   });
+
+  it('keeps an unrelated filled box + ^FR text at a different anchor as two objects', () => {
+    // Anchor mismatch ⇒ no collapse. Hand-written ZPL where a black box
+    // and an ^FR text happen to coexist must round-trip unchanged.
+    const { objects } = parseZPL(
+      '^XA^FO10,10^GB60,30,60,B,0^FS^FO200,200^A0N,30,0^FR^FDHi^FS^XZ',
+      8,
+    );
+    expect(objects).toHaveLength(2);
+    expect(objects[0]?.type).toBe('box');
+    expect(objects[1]?.type).toBe('text');
+    expect(props(objects[1]).reverse).toBe(true);
+  });
 });
 
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