fix(chrome): P5-47 — Mica vs frameless conflict broke custom window chrome

Root cause: lib.rs applies a Mica backdrop to the main window at startup;
set_custom_chrome then stripped the frame with set_decorations(false), and
Mica + frameless is a broken combination on Windows (DWM backdrop glitches
the whole surface).

- set_custom_chrome: clear_mica() before undecorating, re-apply_mica() when
  restoring the native frame; set_shadow(true) so the frameless window keeps
  its drop shadow + resize borders.
- window-state plugin: exclude StateFlags::DECORATIONS — the chrome toggle
  owns the decorated flag; restoring a saved decorated=false at startup would
  recreate the Mica-on-frameless glitch before the web side loads.

Pairs with the web-side TitleBar drag fix (explicit window_start_drag on
mousedown instead of data-tauri-drag-region) in the cinny repo.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -411,7 +411,19 @@ pub fn run() {
             native::toast::show_rich_toast,
         ])
         .plugin(tauri_plugin_localhost::Builder::new(port).build())
-        .plugin(tauri_plugin_window_state::Builder::default().build())
+        .plugin(
+            // DECORATIONS is excluded: the custom-chrome toggle (set_custom_chrome)
+            // owns the decorated flag. Letting window-state restore a saved
+            // decorated=false at startup would re-create the frameless window
+            // BEFORE lib.rs applies Mica (a broken combination) and before the web
+            // side has pushed the user's current setting.
+            tauri_plugin_window_state::Builder::default()
+                .with_state_flags(
+                    tauri_plugin_window_state::StateFlags::all()
+                        & !tauri_plugin_window_state::StateFlags::DECORATIONS,
+                )
+                .build(),
+        )
         .plugin(tauri_plugin_opener::init())
         .plugin(tauri_plugin_notification::init())
         .plugin(tauri_plugin_deep_link::init());

src-tauri/src/native/chrome.rs

@@ -6,11 +6,12 @@
 //! `decorations(true)` and only `set_custom_chrome(true)` makes it frameless, so
 //! the safe default is the untouched native frame.
 //!
-//! Everything here goes through the cross-platform Tauri v2 window API — there is
-//! no `windows` crate dependency, so the same code path runs on Windows, macOS
-//! and Linux. Each command resolves the "main" window and silently no-ops if it
-//! isn't present (e.g. during teardown); the `Result`s are intentionally ignored
-//! since a failed chrome tweak should never surface as an error to the user.
+//! Everything here goes through the cross-platform Tauri v2 window API (plus a
+//! Windows-only `window_vibrancy` dance in `set_custom_chrome`, since Mica and a
+//! frameless window can't coexist). Each command resolves the "main" window and
+//! silently no-ops if it isn't present (e.g. during teardown); the `Result`s are
+//! intentionally ignored since a failed chrome tweak should never surface as an
+//! error to the user.
 
 use tauri::{AppHandle, Manager};
 
@@ -21,7 +22,23 @@ use tauri::{AppHandle, Manager};
 #[tauri::command]
 pub fn set_custom_chrome(app: AppHandle, enabled: bool) {
     if let Some(window) = app.get_webview_window("main") {
+        // Windows: the Mica backdrop (applied at startup in lib.rs) and a
+        // frameless window are a known-bad combo — stripping WS_CAPTION under a
+        // system backdrop glitches the whole surface (black/blank window). Drop
+        // the backdrop before undecorating, and restore it together with the
+        // native frame when custom chrome turns off.
+        #[cfg(target_os = "windows")]
+        if enabled {
+            let _ = window_vibrancy::clear_mica(&window);
+        }
         let _ = window.set_decorations(!enabled);
+        // Re-assert the DWM shadow so a frameless window keeps its drop shadow
+        // and resize borders on Windows (no-op / harmless elsewhere).
+        let _ = window.set_shadow(true);
+        #[cfg(target_os = "windows")]
+        if !enabled {
+            let _ = window_vibrancy::apply_mica(&window, Some(true));
+        }
     }
 }