[client] Fix the Windows tray deadlock on re-entrant window creation

The Wails systray runs the left-click handler synchronously inside the
tray window procedure, and creating a window on a running app pumps a
nested Win32 message loop while WebView2 initialises. ensureWindow held
the non-reentrant createMu across that creation, so the second button-up
of a double click re-entered ShowWindow from the pump and blocked the
main thread on its own lock. A goroutine holding createMu while the main
thread pumped, and the Open* dialogs holding mu across NewWithOptions,
Show, Hide and InvokeSync, exposed the same inversion.

WindowManager now serialises creation with a per-slot creating flag and
queues the callers' operations until the window exists, and no Wails call
runs while mu is held. The tray click and second-instance handlers call
ShowWindow off the message loop.

client/ui/services/windowmanager_test.go

@@ -0,0 +1,127 @@
+//go:build !android && !ios && !freebsd && !js
+
+package services
+
+import (
+	"fmt"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+	"github.com/wailsapp/wails/v3/pkg/application"
+)
+
+func newTestWindowManager() *WindowManager {
+	return &WindowManager{
+		creating:   map[string]bool{},
+		pendingOps: map[string][]windowOp{},
+	}
+}
+
+func waitDone(t *testing.T, done <-chan struct{}, msg string) {
+	t.Helper()
+	select {
+	case <-done:
+	case <-time.After(5 * time.Second):
+		t.Fatal(msg)
+	}
+}
+
+func TestWithWindowReusesExistingWindow(t *testing.T) {
+	s := newTestWindowManager()
+	existing := &application.WebviewWindow{}
+	slot := existing
+	factoryCalls := 0
+	var got *application.WebviewWindow
+	created := true
+	s.withWindow(windowMain, &slot, func() *application.WebviewWindow {
+		factoryCalls++
+		return &application.WebviewWindow{}
+	}, func(w *application.WebviewWindow, c bool) {
+		got, created = w, c
+	})
+	require.Equal(t, 0, factoryCalls)
+	require.Same(t, existing, got)
+	require.False(t, created)
+}
+
+func TestWithWindowNilFactoryWithoutWindowSkipsOp(t *testing.T) {
+	s := newTestWindowManager()
+	var slot *application.WebviewWindow
+	opCalls := 0
+	s.withWindow(windowMain, &slot, nil, func(*application.WebviewWindow, bool) {
+		opCalls++
+	})
+	require.Equal(t, 0, opCalls)
+	require.Nil(t, slot)
+}
+
+func TestWithWindowReentrantCallDuringCreationIsQueued(t *testing.T) {
+	s := newTestWindowManager()
+	var slot *application.WebviewWindow
+	factoryCalls := 0
+	var order []string
+	var factory func() *application.WebviewWindow
+	factory = func() *application.WebviewWindow {
+		factoryCalls++
+		// Simulates the Windows message pump re-entering the tray click handler
+		// while WebView2 is still initialising the window being created.
+		s.withWindow(windowMain, &slot, factory, func(_ *application.WebviewWindow, created bool) {
+			order = append(order, fmt.Sprintf("reentrant:%v", created))
+		})
+		return &application.WebviewWindow{}
+	}
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		s.withWindow(windowMain, &slot, factory, func(_ *application.WebviewWindow, created bool) {
+			order = append(order, fmt.Sprintf("outer:%v", created))
+		})
+	}()
+	waitDone(t, done, "withWindow deadlocked on a re-entrant call during creation")
+
+	require.Equal(t, 1, factoryCalls)
+	require.Equal(t, []string{"outer:true", "reentrant:false"}, order)
+	require.NotNil(t, slot)
+	require.Empty(t, s.creating)
+	require.Empty(t, s.pendingOps)
+}
+
+func TestWithWindowConcurrentCallersShareOneCreation(t *testing.T) {
+	s := newTestWindowManager()
+	var slot *application.WebviewWindow
+	factoryEntered := make(chan struct{})
+	release := make(chan struct{})
+	var factoryCalls, opCalls atomic.Int32
+	factory := func() *application.WebviewWindow {
+		factoryCalls.Add(1)
+		close(factoryEntered)
+		<-release
+		return &application.WebviewWindow{}
+	}
+	op := func(*application.WebviewWindow, bool) { opCalls.Add(1) }
+
+	first := make(chan struct{})
+	go func() {
+		defer close(first)
+		s.withWindow(windowSettings, &slot, factory, op)
+	}()
+	<-factoryEntered
+
+	second := make(chan struct{})
+	go func() {
+		defer close(second)
+		s.withWindow(windowSettings, &slot, factory, op)
+	}()
+	waitDone(t, second, "second caller blocked while the window was being created")
+	require.Equal(t, int32(0), opCalls.Load())
+
+	close(release)
+	waitDone(t, first, "creator did not finish")
+
+	require.Equal(t, int32(1), factoryCalls.Load())
+	require.Equal(t, int32(2), opCalls.Load())
+	require.NotNil(t, slot)
+}