feat: automatically show and hide close buttons from tab bar

This implementation shows and hides all close buttons simultaneously
while mouse cursor hovers over them. A delay was added before close
buttons are hidden again to save the user from a jarring experience
when they off-shoot the mouse cursor reaching for the tab bar.

Ideally we'd only display close buttons for the active tab and the tab
currently being hovered. However, the list of tabs is stored in the
protected side of QTabBar (qtabbar_p.h) and we can't make use of it
without re-inventing the wheel.

src/gui/CMakeLists.txt

@@ -66,6 +66,8 @@ set(PROJECT_SOURCES
     knutmain.cpp
     knutstyle.h
     knutstyle.cpp
+    knuttabwidget.h
+    knuttabwidget.cpp
     logpanel.h
     logpanel.cpp
     mainwindow.h

src/gui/knuttabwidget.cpp

@@ -0,0 +1,56 @@
+/*
+  This file is part of Knut.
+
+SPDX-FileCopyrightText: 2025 Klarälvdalens Datakonsult AB, a KDAB Group company <[email protected]>
+
+SPDX-License-Identifier: GPL-3.0-only
+
+Contact KDAB at <[email protected]> for commercial licensing options.
+*/
+
+#include "knuttabwidget.h"
+
+#include <QEvent>
+#include <QTabBar>
+#include <QTimer>
+
+namespace Gui {
+
+KnutTabBar::KnutTabBar(QWidget *parent)
+    : QTabBar(parent)
+    , m_hideCloseTimer(new QTimer(this))
+{
+    // Human moment delay before hiding close buttons
+    m_hideCloseTimer->setInterval(2000);
+    connect(m_hideCloseTimer, &QTimer::timeout, this, [this]() {
+        setTabsClosable(false);
+    });
+}
+
+bool KnutTabBar::event(QEvent *event)
+{
+    switch (event->type()) {
+    case QEvent::HoverEnter:
+        m_hideCloseTimer->stop();
+        setTabsClosable(true);
+        break;
+    case QEvent::HoverLeave:
+        m_hideCloseTimer->start();
+        break;
+    case QEvent::Show:
+        setTabsClosable(false);
+        break;
+    default:
+        break;
+    }
+    return QTabBar::event(event);
+}
+
+KnutTabWidget::KnutTabWidget(QWidget *parent)
+    : QTabWidget(parent)
+{
+    auto tabBar = new KnutTabBar(this);
+    setTabBar(tabBar);
+}
+
+} // namespace Gui

src/gui/knuttabwidget.h

@@ -0,0 +1,40 @@
+/*
+  This file is part of Knut.
+
+SPDX-FileCopyrightText: 2025 Klarälvdalens Datakonsult AB, a KDAB Group company <[email protected]>
+
+SPDX-License-Identifier: GPL-3.0-only
+
+Contact KDAB at <[email protected]> for commercial licensing options.
+*/
+
+#pragma once
+
+#include <QObject>
+#include <QTabBar>
+#include <QTabWidget>
+#include <QWidget>
+
+namespace Gui {
+
+class KnutTabBar : public QTabBar
+{
+    Q_OBJECT
+public:
+    KnutTabBar(QWidget *parent);
+
+protected:
+    bool event(QEvent *event);
+
+private:
+    QTimer *m_hideCloseTimer;
+};
+
+class KnutTabWidget : public QTabWidget
+{
+    Q_OBJECT
+public:
+    KnutTabWidget(QWidget *parent);
+};
+
+} // namespace Gui

src/gui/mainwindow.ui

@@ -31,7 +31,7 @@
      <number>0</number>
     </property>
     <item>
-     <widget class="QTabWidget" name="tabWidget">
+     <widget class="KnutTabWidget" name="tabWidget">
       <property name="documentMode">
        <bool>true</bool>
       </property>