fix: give session action menu an opaque backdrop

src/session-sidebar-provider.ts

@@ -729,7 +729,12 @@ export class PiSessionSidebarProvider implements vscode.WebviewViewProvider {
       padding: 4px;
       border: 1px solid var(--pi-border);
       border-radius: 4px;
-      background: var(--pi-panel);
+      /* Widget background keeps the menu readable on top of long session
+         lists; sideBarSectionHeader-background is transparent in several
+         themes. The color-mix layer adds a slight translucency so the
+         covered rows stay faintly visible through the backdrop. */
+      background: var(--vscode-editorWidget-background, var(--pi-bg));
+      background: color-mix(in srgb, var(--pi-bg) 94%, transparent);
       box-shadow: 0 6px 18px rgb(0 0 0 / 28%);
     }
     .session-menu.context-positioned {

src/test/session-menu-backdrop.test.ts

@@ -0,0 +1,38 @@
+import * as assert from "node:assert";
+import { readFileSync } from "node:fs";
+
+const source = readFileSync(
+  new URL("../../src/session-sidebar-provider.ts", import.meta.url),
+  "utf8",
+);
+
+function menuRule(): string {
+  const start = source.indexOf(".session-menu {\n");
+  assert.ok(start >= 0, ".session-menu rule not found");
+  const end = source.indexOf("\n    }", start);
+  assert.ok(end > start, ".session-menu rule has no closing brace");
+  return source.slice(start, end + 6);
+}
+
+suite("Session action menu backdrop", () => {
+  test("gives the menu a non-transparent theme-aware background", () => {
+    const menu = menuRule();
+    assert.match(
+      menu,
+      /background: var\(--vscode-editorWidget-background, var\(--pi-bg\)\);/,
+      "expected an opaque widget-background fallback",
+    );
+    assert.match(
+      menu,
+      /background: color-mix\(in srgb, var\(--pi-bg\) 94%, transparent\);/,
+      "expected a near-opaque color-mix backdrop",
+    );
+    assert.doesNotMatch(menu, /var\(--pi-panel\)/, "must not rely on the possibly-transparent section-header token");
+  });
+
+  test("keeps the elevated shadow and border", () => {
+    const menu = menuRule();
+    assert.match(menu, /border: 1px solid var\(--pi-border\)/);
+    assert.match(menu, /box-shadow: 0 6px 18px rgb\(0 0 0 \/ 28%\)/);
+  });
+});