fix(linux): paint a hairline edge on the frameless settings window

Linux compositors draw no shadow for undecorated windows (Wayland has
no server-side decorations at all), so the frameless settings window is
indistinguishable from the desktop there — #595's follow-up after the
Windows DWM-shadow fix. Paint a 1px --border-window inset ring around
the window root: the webview draws the same hairline on every
windowing system.

src/features/settings/__tests__/dialog.test.tsx

@@ -22,6 +22,7 @@ vi.mock("@/platform/ipc", async (importOriginal) => {
 });
 
 import { registerAction, executeAction } from "@/platform/actions";
+import { usePlatform } from "@/platform/platformInfo";
 import { useProject } from "@/platform/project";
 import { useKeymap } from "@/platform/keymap";
 import { ContextMenuHost } from "@/ui";
@@ -29,6 +30,8 @@ import { PageContext, useSetting } from "../components/context";
 import { DEFAULT_SETTINGS, setAt } from "../model/schema";
 import { openSettings, stage, useSettingsDialog } from "../model/staging";
 import { SettingsDialog } from "../dialog/SettingsDialog";
+import { SettingsWindow } from "../window/SettingsWindow";
+import dialogStyles from "../dialog/SettingsDialog.module.css";
 import { activate } from "..";
 import { MessagesHost } from "@/workbench/messages";
 
@@ -471,6 +474,23 @@ describe("Settings dialog", () => {
   });
 });
 
+describe("native Settings window root", () => {
+  it("marks the window edge on Linux, whose frameless windows get no compositor shadow", () => {
+    const saved = usePlatform.getState();
+    usePlatform.setState({ os: "linux" });
+    let view = render(<SettingsWindow />);
+    let root = document.querySelector(`.${dialogStyles.window}`);
+    expect(root).toHaveClass(dialogStyles.windowEdge!);
+    view.unmount();
+    usePlatform.setState({ os: "windows" });
+    view = render(<SettingsWindow />);
+    root = document.querySelector(`.${dialogStyles.window}`);
+    expect(root).not.toHaveClass(dialogStyles.windowEdge!);
+    view.unmount();
+    usePlatform.setState(saved);
+  });
+});
+
 describe("native Settings window background", () => {
   it.each([
     ["a hex frame", "#26282c", "", "#26282c"],

src/features/settings/dialog/SettingsDialog.module.css

@@ -203,6 +203,11 @@
   background: var(--bg-dialog, var(--bg-elev));
   color: var(--fg);
 }
+/* Linux draws no shadow for a frameless window (Wayland/KDE give undecorated
+   windows no CSD), so mark its edge in CSS instead. */
+.windowEdge {
+  box-shadow: inset 0 0 0 1px var(--border-window);
+}
 .windowBody {
   flex: 1;
   min-height: 0;

src/features/settings/window/SettingsWindow.tsx

@@ -1,4 +1,5 @@
 /** Content of the native Settings window: title bar, page tree + page, buttons. */
+import clsx from "clsx";
 import { useEffect, useRef } from "react";
 import { OverlayHost } from "@/workbench/OverlayHost";
 import { usePlatform } from "@/platform/platformInfo";
@@ -10,6 +11,7 @@ import styles from "../dialog/SettingsDialog.module.css";
 
 export function SettingsWindow() {
   useApplyTheme();
+  const os = usePlatform((s) => s.os);
   const ownTitleBar = usePlatform((s) => s.overlayTitleBar || s.framelessTitleBar);
   const open = useSettingsDialog((s) => s.open);
   const title = useSettingsTitle();
@@ -20,8 +22,8 @@ export function SettingsWindow() {
   }, [open]);
 
   return (
-    <div ref={ref} className={styles.window}>
-      {/* macOS overlays traffic lights on it; Windows is frameless. Linux keeps the OS title bar. */}
+    <div ref={ref} className={clsx(styles.window, os === "linux" && styles.windowEdge)}>
+      {/* macOS overlays traffic lights on it; Windows and Linux are frameless (tauri.<os>.conf.json). */}
       {ownTitleBar && <TitleBar minimal dialog title={title} />}
       {open && (
         <>