fix: blend the canvas background over black

The opacity slider is documented as a blend over black, and the preview
image does that. The editor composited the image over #1a1a1a instead.

tests/caption_layout.test.js

@@ -2,6 +2,7 @@ import assert from "node:assert/strict";
 import { describe, it } from "node:test";
 
 import {
+  backgroundPlateShown,
   captionText,
   cleanPalette,
   elementToRegion,
@@ -236,6 +237,16 @@ describe("overlapping regions", () => {
   });
 });
 
+describe("background image plate", () => {
+  it("is black only while a reference image is actually drawn", () => {
+    assert.equal(backgroundPlateShown("/view?filename=bg.png", 0.5), true);
+    assert.equal(backgroundPlateShown("/view?filename=bg.png", 1), true);
+    assert.equal(backgroundPlateShown("/view?filename=bg.png", 0), false);
+    assert.equal(backgroundPlateShown("", 0.5), false);
+    assert.equal(backgroundPlateShown(null, 0.5), false);
+  });
+});
+
 describe("color palettes", () => {
   it("uppercases #RRGGBB, expands #RGB, and drops values Ideogram rejects", () => {
     assert.deepEqual(

web/js/caption_layout.js

@@ -228,6 +228,15 @@ export function cleanPalette(colors, limit) {
   return out;
 }
 
+/**
+ * The reference image is drawn with CSS opacity. The black plate is what
+ * that opacity blends over, matching the preview. Hidden when there is no
+ * image, so the empty canvas stays the editor gray.
+ */
+export function backgroundPlateShown(url, opacity) {
+  return Boolean(url) && Number(opacity) > 0;
+}
+
 /** Lowercase #rrggbb for input[type=color]. Anything else is unusable there. */
 export function swatchHex(color) {
   const hex = normalizeHex(color);

web/js/ideogram4_prompt_builder_enhanced.js

@@ -8,6 +8,7 @@ import {
   normalizeStoreName, savedPresetCollision,
 } from "./ideogram4_shared.js";
 import {
+  backgroundPlateShown,
   captionText, cleanPalette, dimsFromAspect, elementToRegion, ideogramResolutionWarning,
   parseCaptionText,
   snapLayoutDim,
@@ -129,6 +130,7 @@ function injectStyle() {
   s.textContent = `
     .byae-ideo-wrap { display:flex; flex-direction:column; overflow:hidden; position:relative; pointer-events:auto; gap:4px; width:100%; box-sizing:border-box; }
     .byae-ideo-canvas-wrap { position:relative; flex:0 0 auto; width:100%; border-radius:4px; overflow:hidden; background:#1a1a1a; }
+    .byae-ideo-bg-plate { position:absolute; inset:0; background:#000; pointer-events:none; display:none; }
     .byae-ideo-bg { position:absolute; inset:0; width:100%; height:100%; object-fit:fill; pointer-events:none; display:none; }
     .byae-ideo-canvas { cursor:crosshair; display:block; width:100%; height:auto; position:relative; z-index:1; background:transparent; border-radius:4px; outline:none; }
     .byae-ideo-bar { display:flex; align-items:center; gap:6px; font:11px sans-serif; color:#aaa; user-select:none; padding:0 2px; flex:0 0 auto; flex-wrap:wrap; width:100%; box-sizing:border-box; }
@@ -753,6 +755,8 @@ app.registerExtension({
 
       const canvasWrap = document.createElement("div");
       canvasWrap.className = "byae-ideo-canvas-wrap";
+      const bgPlate = document.createElement("div");
+      bgPlate.className = "byae-ideo-bg-plate";
       const bgImgEl = document.createElement("img");
       bgImgEl.className = "byae-ideo-bg";
       bgImgEl.alt = "";
@@ -766,6 +770,7 @@ app.registerExtension({
       const canvasEl = document.createElement("canvas");
       canvasEl.className = "byae-ideo-canvas";
       canvasEl.tabIndex = 0;
+      canvasWrap.appendChild(bgPlate);
       canvasWrap.appendChild(bgImgEl);
       canvasWrap.appendChild(canvasEl);
       let ctx = canvasEl.getContext("2d");
@@ -2233,11 +2238,16 @@ app.registerExtension({
       }
 
       function syncBgDisplay() {
+        const host = getEditorHost();
         const { bgImgEl: bg } = getLiveCanvas();
+        const plate = host?.querySelector?.(".byae-ideo-bg-plate");
         const url = node._bgDisplayUrl || "";
         const op = getBgOpacity();
         if (!bg) return;
-        if (url && op > 0) {
+        // Opacity blends the image with whatever is painted behind it. The
+        // plate is black so that matches the preview, which composites onto black.
+        const show = backgroundPlateShown(url, op);
+        if (show) {
           if (bg.dataset.byaeSrc !== url) {
             bg.dataset.byaeSrc = url;
             bg.src = url;
@@ -2249,6 +2259,7 @@ app.registerExtension({
           bg.removeAttribute("src");
           bg.style.display = "none";
         }
+        if (plate) plate.style.display = show ? "block" : "none";
       }
 
       function applyBgDisplay(displayUrl) {