fix(windows): restore taskbar thumbnail media buttons after deferred window show (#1112)

* fix(windows): restore taskbar thumbnail media buttons after deferred window show

The Prev / Play-Pause / Next buttons in the Windows taskbar thumbnail
preview stopped appearing. The taskbar code was unchanged; the regression
came from the main window now starting hidden with a deferred show. ThumbBarAddButtons was still called at setup time, before the shell had
created the window's taskbar button, so it returned S_OK but added
nothing (no error logged).

Register the shell's "TaskbarButtonCreated" message and add the buttons
from the window subclass when it fires (first show, and again after an
explorer restart), which is the documented requirement for ThumbBarAddButtons.

* docs(changelog): note Windows taskbar media buttons fix (#1112)

src-tauri/src/taskbar_win.rs

@@ -12,10 +12,11 @@
 //! strict.
 #![cfg_attr(debug_assertions, allow(dead_code))]
 
-use std::sync::atomic::{AtomicIsize, Ordering};
+use std::sync::atomic::{AtomicIsize, AtomicU32, Ordering};
 
 use tauri::{AppHandle, Emitter};
 use windows::{
+    core::w,
     Win32::{
         Foundation::{HWND, LPARAM, LRESULT, WPARAM},
         System::Com::{
@@ -29,7 +30,7 @@ use windows::{
             },
             WindowsAndMessaging::{
                 CreateIconFromResourceEx, DestroyIcon, HICON, LR_DEFAULTCOLOR,
-                WM_COMMAND, WM_NCDESTROY,
+                RegisterWindowMessageW, WM_COMMAND, WM_NCDESTROY,
             },
         },
     },
@@ -61,6 +62,10 @@ static HICON_PLAY:  AtomicIsize = AtomicIsize::new(0);
 static HICON_PAUSE: AtomicIsize = AtomicIsize::new(0);
 static HICON_NEXT:  AtomicIsize = AtomicIsize::new(0);
 
+// Registered window-message id for the shell's "TaskbarButtonCreated"
+// broadcast (0 until registered in `init`).
+static TASKBAR_BUTTON_CREATED_MSG: AtomicU32 = AtomicU32::new(0);
+
 // ── ICO resource loader ──────────────────────────────────────────────────────
 
 /// Load the best-match image from a raw `.ico` file in memory and return an HICON.
@@ -174,6 +179,15 @@ unsafe extern "system" fn subclass_proc(
     _uid:   usize,
     data:   usize,
 ) -> LRESULT {
+    // The shell sends this once the taskbar button exists (on the first window
+    // show) and again after an explorer.exe restart — the only safe moment to
+    // (re)add the thumbnail buttons. See `add_thumb_buttons`.
+    let tb_created = TASKBAR_BUTTON_CREATED_MSG.load(Ordering::SeqCst);
+    if tb_created != 0 && msg == tb_created {
+        add_thumb_buttons();
+        return DefSubclassProc(hwnd, msg, wparam, lparam);
+    }
+
     if msg == WM_COMMAND {
         let hi = (wparam.0 >> 16) as u32;
         let lo = (wparam.0 & 0xFFFF) as u32;
@@ -211,6 +225,43 @@ unsafe extern "system" fn subclass_proc(
     DefSubclassProc(hwnd, msg, wparam, lparam)
 }
 
+// ── Thumbnail-button (re)attach ────────────────────────────────────────────────
+
+/// (Re)add the three media buttons to the taskbar thumbnail toolbar.
+///
+/// Must run only after the shell has created the window's taskbar button and
+/// broadcast `TaskbarButtonCreated`. Calling `ThumbBarAddButtons` before that
+/// point returns `S_OK` but silently adds nothing — which is why the buttons
+/// disappeared once the main window started hidden/deferred (PR #1030 moved the
+/// window to `visible: false` with a deferred show, so at `setup` time the
+/// taskbar button does not exist yet).
+unsafe fn add_thumb_buttons() {
+    let taskbar_raw = TASKBAR_PTR.load(Ordering::SeqCst);
+    let hwnd_raw    = HWND_VAL.load(Ordering::SeqCst);
+    if taskbar_raw == 0 || hwnd_raw == 0 { return; }
+
+    let h_prev = HICON_PREV.load(Ordering::SeqCst);
+    let h_play = HICON_PLAY.load(Ordering::SeqCst);
+    let h_next = HICON_NEXT.load(Ordering::SeqCst);
+    if h_prev == 0 || h_play == 0 || h_next == 0 { return; }
+
+    let taskbar = &*(taskbar_raw as *const ITaskbarList3);
+    let hwnd    = HWND(hwnd_raw as *mut _);
+
+    // Harmless on an already-initialised object; required again after an
+    // explorer restart recreates the taskbar button.
+    let _ = taskbar.HrInit();
+
+    let buttons = make_buttons(
+        HICON(h_prev as *mut _),
+        HICON(h_play as *mut _),
+        HICON(h_next as *mut _),
+    );
+    if let Err(e) = taskbar.ThumbBarAddButtons(hwnd, &buttons) {
+        crate::app_eprintln!("[psysonic] taskbar: ThumbBarAddButtons failed: {e}");
+    }
+}
+
 // ── Public init ──────────────────────────────────────────────────────────────
 
 pub fn init(app: &AppHandle, hwnd_raw: isize) {
@@ -242,11 +293,12 @@ pub fn init(app: &AppHandle, hwnd_raw: isize) {
         HICON_PAUSE.store(h_pause.0 as isize, Ordering::SeqCst);
         HICON_NEXT .store(h_next .0 as isize, Ordering::SeqCst);
 
-        let buttons = make_buttons(h_prev, h_play, h_next);
-        if let Err(e) = taskbar.ThumbBarAddButtons(hwnd, &buttons) {
-            crate::app_eprintln!("[psysonic] taskbar: ThumbBarAddButtons failed: {e}");
-            return;
-        }
+        // Register the shell's "TaskbarButtonCreated" message. The buttons are
+        // added from the subclass proc when it fires (after the first window
+        // show), because the deferred/hidden main window means the taskbar
+        // button does not exist yet at this point.
+        let tb_msg = RegisterWindowMessageW(w!("TaskbarButtonCreated"));
+        TASKBAR_BUTTON_CREATED_MSG.store(tb_msg, Ordering::SeqCst);
 
         let raw = Box::into_raw(Box::new(taskbar));
         TASKBAR_PTR.store(raw as isize, Ordering::SeqCst);