resize: reads before writes everywhere, and an open album drags like a closed one

src/lib/heroGpu.js

@@ -535,10 +535,26 @@ export function createHeroGpuRenderer(canvas, bloomCanvas, previousCanvas, previ
         );
         const width = Math.max(1, Math.round(rect.width * scale));
         const height = Math.max(1, Math.round(rect.height * scale));
-        if (canvas.width !== width) canvas.width = width;
-        if (canvas.height !== height) canvas.height = height;
+        // A store within a couple of percent of the box is not visibly softer —
+        // that is the same argument reflow() already makes for every
+        // intermediate frame of a drag, and it holds just as well for the final
+        // one. Without the tolerance, ANY change reallocates: measured two
+        // ~52ms hitches landing at 1827ms and 1909ms against a drag that ended
+        // at 1823ms, i.e. the whole cost of a resize arriving after the user
+        // had stopped. paint() below is geometrically exact at any store size,
+        // so skipping the realloc costs nothing but sharpness we cannot see.
+        const REALLOC_TOLERANCE = 0.02;
+        const off = (a, b) => Math.abs(a - b) / Math.max(1, b) > REALLOC_TOLERANCE;
+        if (off(canvas.width, width) || off(canvas.height, height)) {
+            if (canvas.width !== width) canvas.width = width;
+            if (canvas.height !== height) canvas.height = height;
+        }
         paint(rect);
-        gl.finish();
+        // gl.finish() blocks the main thread until the GPU has drained, and it
+        // stalls hardest exactly when the GPU is busy — which, 140ms after a
+        // drag, it still is. Nothing here reads back, so there is nothing to
+        // synchronise with; the compositor will show the frame when it is done.
+        gl.flush();
     }
 
     /** The ResizeObserver's path, and the reason it is not `resize`.