fix(web): refine sidebar spinner and backdrop compositing

apps/web/src/components/Sidebar.tsx

@@ -236,22 +236,21 @@ function JumpHintBadge(props: { label: string }) {
 // lucide's CircleCheck (r=10, edge 11) sat visibly inside the spinner.
 //
 // The spinner is Material's transform-only construction (see .working-spinner
-// in index.css): a static track ring, then two half-ring SVGs whose rotation
-// inside clipped halves makes the arc grow and reel in without animating any
-// paint property. Half ring = full circumference dash, offset by half.
-const SPINNER_RING_CIRCUMFERENCE = 2 * Math.PI * 10;
-
-function SpinnerHalfRing() {
+// in index.css): a static track ring, then two filled half-ring paths whose
+// rotation inside clipped halves makes the arc grow and reel in without
+// animating any paint property. Each path is one continuous silhouette with a
+// square clipped-seam end and a rounded exposed end, avoiding both hidden-cap
+// leakage and self-overdraw between a stroked path and a separate cap shape.
+function SpinnerHalfRing({ roundedEnd }: { roundedEnd: "start" | "end" }) {
   return (
     <svg aria-hidden xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24">
-      <circle
-        cx="12"
-        cy="12"
-        r="10"
-        stroke="currentColor"
-        strokeWidth="4"
-        strokeDasharray={SPINNER_RING_CIRCUMFERENCE}
-        strokeDashoffset={SPINNER_RING_CIRCUMFERENCE / 2}
+      <path
+        d={
+          roundedEnd === "start"
+            ? "M24 12A12 12 0 0 1 0 12L4 12A8 8 0 0 0 20 12A2 2 0 0 1 24 12Z"
+            : "M24 12A12 12 0 0 1 0 12A2 2 0 0 1 4 12A8 8 0 0 0 20 12Z"
+        }
+        fill="currentColor"
       />
     </svg>
   );
@@ -271,13 +270,10 @@ function WorkingSpinnerIcon(props: { className?: string }) {
       <span className="working-spinner__container">
         <span className="working-spinner__layer">
           <span className="working-spinner__clipper working-spinner__clipper--left">
-            <SpinnerHalfRing />
-          </span>
-          <span className="working-spinner__patch">
-            <SpinnerHalfRing />
+            <SpinnerHalfRing roundedEnd="start" />
           </span>
           <span className="working-spinner__clipper working-spinner__clipper--right">
-            <SpinnerHalfRing />
+            <SpinnerHalfRing roundedEnd="end" />
           </span>
         </span>
       </span>

apps/web/src/index.css

@@ -406,12 +406,13 @@ html[data-mobile-composer-route-transition="true"]::view-transition-old(t3-mobil
     var(--app-scrollbar-width) 100%;
 }
 
-/* Stage-channel art needs a mask and pseudo-element gradient, so keep the
-   behavior composable without tying it to the global components layer. */
+/* Fade stage-channel art with an overlay rather than a CSS mask. Chromium can
+   mis-composite the masked blueprint SVG while another sidebar layer is
+   animating, leaking fragments of the artwork into unrelated tiles. The
+   overlay reaches the sidebar surface color by 93%, so the mask was visually
+   redundant as well as an extra composited layer. */
 @utility sidebar-stage-backdrop {
   --stage-fade: var(--sidebar-stage-fade, var(--app-chrome-background));
-  mask-image: linear-gradient(to bottom, black 0%, black 55%, transparent 92%);
-  -webkit-mask-image: linear-gradient(to bottom, black 0%, black 55%, transparent 92%);
 
   &::after {
     content: "";
@@ -1904,13 +1905,13 @@ code {
 }
 
 /* Sidebar "Working" spinner: Material's indeterminate ring, transform-only.
-   The arc's grow-and-reel-in is faked by two half-ring SVGs, one per clipped
-   half, counter-rotating with ease-in-out inside a stepped-rotation layer
-   inside a linearly rotating container (MDC's construction and timings). Every
-   animated property is a transform on an HTML element or a root <svg>, so the
-   compositor drives all of it and the main thread never repaints a frame —
-   the same bar the stepped status indicators hold themselves to. A stroke-dash
-   animation would look identical but repaint on the main thread every vsync. */
+   The arc's grow-and-reel-in is faked by two filled half-ring SVGs, one per
+   clipped half, counter-rotating with ease-in-out inside a stepped-rotation
+   layer inside a linearly rotating container (MDC's construction and timings).
+   Every animated property is a transform on an HTML element or a root <svg>,
+   so the compositor drives all of it and the main thread never repaints a
+   frame. A stroke-dash animation would look identical but repaint on the main
+   thread every vsync. */
 .working-spinner {
   position: relative;
   display: block;
@@ -1974,27 +1975,6 @@ code {
   animation: working-spinner-right-spin 1333ms cubic-bezier(0.4, 0, 0.2, 1) infinite both;
 }
 
-/* A hairline sliver of ring parked over the 12 o'clock seam where the two
-   halves meet (the arc always spans it), so antialiasing never shows a gap. */
-.working-spinner__patch {
-  position: absolute;
-  top: 0;
-  left: 46%;
-  display: block;
-  width: 8%;
-  height: 100%;
-  overflow: hidden;
-}
-
-.working-spinner__patch > svg {
-  position: absolute;
-  top: 0;
-  left: -575%;
-  width: 1250%;
-  height: 100%;
-  transform: rotate(180deg);
-}
-
 @keyframes working-spinner-container-rotate {
   to {
     transform: rotate(360deg);