Fix Qt widget embedding: pages rendered as floating windows instead of tabs

Move centralWidget assignment before page creation to fix use of
uninitialized pointer. Use Qt::Widget flags when pages have a parent
(embedded in QStackedWidget) and pass centralWidget as parent for all
lazily-created pages (messagePage, signMessagePage, verifyMessagePage).
Also fix TransactionView which unconditionally set FramelessWindowHint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/qt/addressbookpage.cpp

@@ -27,7 +27,10 @@ AddressBookPage::AddressBookPage(Mode mode, Tabs tab, QWidget *parent) :
     tab(tab)
 {
     ui->setupUi(this);
-    setWindowFlags(Qt::CustomizeWindowHint | Qt::FramelessWindowHint | Qt::Window);
+    if (mode == ForEditing && parent)
+        setWindowFlags(Qt::Widget);
+    else
+        setWindowFlags(Qt::CustomizeWindowHint | Qt::FramelessWindowHint | Qt::Window);
     ui->wAddressBookHeader->installEventFilter(new DialogMoveHandler(this));
 
 #ifdef Q_OS_MAC // Icons on push buttons are very uncommon on Mac

src/qt/sendcoinsdialog.cpp

@@ -28,7 +28,10 @@ SendCoinsDialog::SendCoinsDialog(QWidget *parent) :
     model(0)
 {
     ui->setupUi(this);
-    setWindowFlags(Qt::CustomizeWindowHint | Qt::FramelessWindowHint | Qt::Window);
+    if (parent)
+        setWindowFlags(Qt::Widget);
+    else
+        setWindowFlags(Qt::CustomizeWindowHint | Qt::FramelessWindowHint | Qt::Window);
 
 #ifdef Q_OS_MAC // Icons on push buttons are very uncommon on Mac
     ui->addButton->setIcon(QIcon());

src/qt/transactionview.cpp

@@ -40,7 +40,10 @@ TransactionView::TransactionView(QWidget *parent) :
     transactionSortOrder(Qt::DescendingOrder)
 {
     ui->setupUi(this);
-    setWindowFlags(Qt::CustomizeWindowHint | Qt::FramelessWindowHint | Qt::Window);
+    if (parent)
+        setWindowFlags(Qt::Widget);
+    else
+        setWindowFlags(Qt::CustomizeWindowHint | Qt::FramelessWindowHint | Qt::Window);
     // Build filter row
     dateWidget = ui->dateWidget;
     dateWidget->addItem(tr("All"), All);

src/qt/trianglesgui.cpp

@@ -284,6 +284,7 @@ TrianglesGUI::TrianglesGUI(bool fIsTestnet, QWidget *parent):
     createTrayIcon();
 
     // Create tabs
+    centralWidget = ui->stackedWidget;
     overviewPage = new OverviewPage();
     {
       transactionsPage = new QWidget(this);
@@ -296,15 +297,14 @@ TrianglesGUI::TrianglesGUI(bool fIsTestnet, QWidget *parent):
       vbox->addWidget(transactionView);
       frameMain->setLayout(vbox);
     }
-    addressBookPage = new AddressBookPage(AddressBookPage::ForEditing, AddressBookPage::SendingTab);
+    addressBookPage = new AddressBookPage(AddressBookPage::ForEditing, AddressBookPage::SendingTab, centralWidget);
 
-    receiveCoinsPage = new AddressBookPage(AddressBookPage::ForEditing, AddressBookPage::ReceivingTab);
+    receiveCoinsPage = new AddressBookPage(AddressBookPage::ForEditing, AddressBookPage::ReceivingTab, centralWidget);
 
     sendCoinsPage = 0;
     messagePage = 0;
 	signMessagePage = 0;
     verifyMessagePage = 0;
-    centralWidget = ui->stackedWidget;
     centralWidget->addWidget(overviewPage);
     centralWidget->addWidget(transactionsPage);
     centralWidget->addWidget(addressBookPage);
@@ -641,7 +641,7 @@ void TrianglesGUI::ensureSendCoinsPage()
     if (sendCoinsPage)
         return;
 
-    sendCoinsPage = new SendCoinsDialog(this);
+    sendCoinsPage = new SendCoinsDialog(centralWidget);
     if (walletModel)
         sendCoinsPage->setModel(walletModel);
     centralWidget->addWidget(sendCoinsPage);
@@ -652,7 +652,7 @@ void TrianglesGUI::ensureMessagePage()
     if (messagePage)
         return;
 
-    messagePage = new MessagePage(this);
+    messagePage = new MessagePage(centralWidget);
     if (messageModel)
         messagePage->setModel(messageModel);
     centralWidget->addWidget(messagePage);
@@ -663,7 +663,7 @@ void TrianglesGUI::ensureSignMessagePage()
     if (signMessagePage)
         return;
 
-    signMessagePage = new SignMessagePage(this);
+    signMessagePage = new SignMessagePage(centralWidget);
     if (walletModel)
         signMessagePage->setModel(walletModel);
     centralWidget->addWidget(signMessagePage);
@@ -674,7 +674,7 @@ void TrianglesGUI::ensureVerifyMessagePage()
     if (verifyMessagePage)
         return;
 
-    verifyMessagePage = new VerifyMessagePage(this);
+    verifyMessagePage = new VerifyMessagePage(centralWidget);
     if (walletModel)
         verifyMessagePage->setModel(walletModel);
     centralWidget->addWidget(verifyMessagePage);