QMargins QAbstractScrollArea::viewportMargins()

QMargins QAbstractScrollArea::viewportMargins() とは

QMargins QAbstractScrollArea::viewportMargins() は、Qtの QAbstractScrollArea クラスのメソッドです。このメソッドは、スクロール領域のビューポートの周囲に設定されたマージン（余白）の大きさを QMargins オブジェクトとして返します。

QAbstractScrollArea とは

QAbstractScrollArea は、スクロール可能な領域を提供するウィジェットの基底クラスです。これは、ビューポートと呼ばれる中心のウィジェットを持ち、その中にコンテンツが表示され、必要に応じてスクロールバーが表示されます。QScrollArea や QTableView、QTreeView など、多くのスクロール可能なウィジェットがこのクラスを継承しています。

QMargins とは

QMargins クラスは、長方形の4辺（左、上、右、下）のサイズを保持するクラスです。viewportMargins() メソッドが返す QMargins オブジェクトには、ビューポートの各辺にどれだけの余白が設定されているかが含まれています。

viewportMargins() の役割

viewportMargins() メソッドが返すマージンは、主に以下の目的で使用されます。

1. カスタムスクロールエリアの実装: QAbstractScrollArea を直接継承して独自のスクロールエリアを実装する場合、viewportMargins() を使って現在のマージンを取得し、それに基づいてビューポートのレイアウトやコンテンツの描画位置を調整することができます。

例えば、QTableView のようなウィジェットでは、カラムヘッダーや行ヘッダーがスクロールするコンテンツとは別に表示されます。これらのヘッダーは、QAbstractScrollArea のビューポートの上下左右のマージンとして考慮され、そのスペースに描画されます。

---

setViewportMargins() の効果がない

よくあるエラー

• QAbstractScrollArea を直接継承して独自のスクロールエリアを作成したが、setViewportMargins() を設定してもビューポートの表示が正しく変わらない。
• QTableView や QTreeView などのQtが提供する標準のビュークラスで setViewportMargins() を呼び出しても、見た目に変化がない、または期待通りのマージンが適用されない。

原因とトラブルシューティング

• カスタムスクロールエリアでのレイアウトの欠如
  QAbstractScrollArea を継承してカスタムのスクロールエリアを作成した場合、setViewportMargins() を設定しただけでは、そのマージン領域に何かを表示するためのレイアウトや描画ロジックが自動的に追加されるわけではありません。マージン領域は「空きスペース」として確保されるだけです。

  • 対策
    
    • 手動でのウィジェット配置
      QAbstractScrollArea の resizeEvent() をオーバーライドし、viewportMargins() で取得したマージン情報に基づいて、マージン領域に配置したいウィジェット（例えば、カスタムのヘッダーウィジェットなど）の位置とサイズを手動で計算して move() や resize() で設定する必要があります。
    • viewport() の利用
      ビューポートの親ウィジェット（QAbstractScrollArea 自身）に、マージン領域を含んだ全体のレイアウトを設定し、ビューポートをそのレイアウトの一部として配置する方法も考えられます。ただし、この場合、ビューポートのマージンとしてではなく、親ウィジェットのレイアウトのマージンとして扱われます。
    • QScrollArea の検討
      複雑なレイアウトが必要ない場合や、単純なウィジェットをスクロールさせたいだけであれば、QAbstractScrollArea の代わりに QScrollArea を使用することを検討してください。QScrollArea は内部でウィジェットのスクロールを処理するため、setWidget() で子ウィジェットを設定するだけで済み、マージンの複雑な処理は不要になることが多いです。

• 標準ビュークラスの内部管理
  QTableView や QTreeView など、Qtの多くの標準ビュークラスは、内部で自身のヘッダー（QHeaderView）の配置を管理するために viewportMargins を使用しています。そのため、これらのクラスに対して setViewportMargins() を外部から呼び出しても、その設定が無視されるか、期待通りの効果が得られないことがあります。これはQtの設計上の挙動であり、バグではありません。

  • 対策
    これらのクラスでヘッダーの位置やサイズを調整したい場合は、直接 QHeaderView のプロパティ（例: setMinimumSectionSize() など）や、モデル/ビューアーキテクチャを通じてビューの表示を制御することを検討してください。

スクロールバーとマージンの競合

• setViewportMargins() でマージンを設定すると、スクロールバーが表示されるべき領域がマージンによって隠れてしまう、またはスクロールバーの表示が不自然になる。

• スクロールバーのポリシー
  QAbstractScrollArea は、horizontalScrollBarPolicy と verticalScrollBarPolicy というプロパティを持っており、スクロールバーの表示ポリシーを制御します（例: Qt::ScrollBarAsNeeded、Qt::ScrollBarAlwaysOn、Qt::ScrollBarAlwaysOff）。これらのポリシーとマージンの設定が競合すると、意図しない表示になることがあります。特に Qt::ScrollBarAsNeeded の場合、コンテンツのサイズとビューポートのサイズに基づいてスクロールバーの表示/非表示が自動的に切り替わるため、マージンの計算に影響を与える可能性があります。
  • 対策
    
    • ポリシーの確認
      スクロールバーの表示ポリシーが適切に設定されているかを確認します。必要に応じて Qt::ScrollBarAlwaysOn を試して、スクロールバーが常に表示されるように強制してみることで、マージンの影響を切り分けることができます。
    • レイアウトの再計算
      マージンやコンテンツのサイズが変更された場合、updateGeometry() や viewport()->updateGeometry() を呼び出して、レイアウトを再計算させる必要がある場合があります。

• 描画された結果と viewportMargins() で取得した値が一致しない。
• viewportMargins() を呼び出すと、setViewportMargins() で設定した値とは異なる値が返ってくる。

• システムスタイルとテーマ
  Qtのスタイルシステムやオペレーティングシステムのテーマによっては、ウィジェットのレンダリングに影響を与え、マージンが視覚的に期待通りに見えないことがあります。

  • 対策
    スタイルシート (setStyleSheet()) を使用して、明示的にパディングやマージンを設定し、スタイルシステムの影響をオーバーライドすることを検討してください。ただし、これはQtの標準的なマージン管理とは別のレイヤーでの話になります。

• デフォルトマージン
  QAbstractScrollArea のサブクラスによっては、デフォルトで何らかのマージンが設定されている場合があります。特に、Qtが提供するビュークラスは、内部で独自のロジックに基づいてマージンを計算・設定することがあります。

  • 対策
    setViewportMargins(0, 0, 0, 0) を呼び出して、一度マージンをリセットし、その後で目的の値を設定してみることで、初期値の影響を排除できる場合があります。

---

例1: カスタムのスクロールエリアと固定ヘッダー

この例では、MyScrollArea というカスタム QAbstractScrollArea を作成し、その上部に固定されたヘッダーウィジェット（QLabel）を配置します。

my_scroll_area.h

#ifndef MY_SCROLL_AREA_H
#define MY_SCROLL_AREA_H

#include <QAbstractScrollArea>
#include <QLabel>
#include <QMargins>
#include <QResizeEvent>
#include <QVBoxLayout> // レイアウト管理にQVBoxLayoutを使用

class MyScrollArea : public QAbstractScrollArea
{
    Q_OBJECT

public:
    explicit MyScrollArea(QWidget *parent = nullptr);

    // ビューポートに表示するコンテンツを設定するメソッド
    void setContentWidget(QWidget *widget);

protected:
    // ビューポートのサイズ変更イベントを処理
    void resizeEvent(QResizeEvent *event) override;
    // スクロールバーの値の変更を処理
    void scrollContentsBy(int dx, int dy) override;

private:
    QLabel *topHeaderLabel; // 上部に表示する固定ヘッダー
    QWidget *contentWidget; // スクロールされるコンテンツ
};

#endif // MY_SCROLL_AREA_H

my_scroll_area.cpp

#include "my_scroll_area.h"
#include <QDebug> // デバッグ出力用

MyScrollArea::MyScrollArea(QWidget *parent)
    : QAbstractScrollArea(parent),
      contentWidget(nullptr)
{
    // ヘッダーウィジェットの作成
    topHeaderLabel = new QLabel("Custom Header", this);
    topHeaderLabel->setStyleSheet("background-color: lightblue; border: 1px solid blue;");
    topHeaderLabel->setAlignment(Qt::AlignCenter);
    topHeaderLabel->setFixedHeight(30); // ヘッダーの高さを固定

    // ビューポートのマージンを設定
    // 上部にヘッダーの高さ分のマージンを設ける
    setViewportMargins(0, topHeaderLabel->height(), 0, 0);

    // スクロールバーのポリシーを設定（必要に応じて表示）
    setHorizontalScrollBarPolicy(Qt::ScrollBarAsNeeded);
    setVerticalScrollBarPolicy(Qt::ScrollBarAsNeeded);
}

void MyScrollArea::setContentWidget(QWidget *widget)
{
    if (contentWidget) {
        contentWidget->setParent(nullptr); // 既存のコンテンツを親から解除
    }
    contentWidget = widget;
    // ビューポートの子ウィジェットとして設定
    // QAbstractScrollArea::setViewport() は QScrollArea::setWidget() と異なり、
    // ビューポートの親ウィジェットがコンテンツを直接管理するわけではないことに注意
    // ここでは、contentWidgetをviewport()の子として設定
    if (contentWidget) {
        contentWidget->setParent(viewport());
        // contentWidgetのサイズポリシーを適切に設定
        contentWidget->setSizePolicy(QSizePolicy::MinimumExpanding, QSizePolicy::MinimumExpanding);
        // コンテンツの最小サイズを反映させる
        viewport()->setMinimumSize(contentWidget->sizeHint());
        // レイアウトを更新
        updateGeometry();
    }
}

void MyScrollArea::resizeEvent(QResizeEvent *event)
{
    QAbstractScrollArea::resizeEvent(event); // 親クラスのイベントを呼び出す

    // ビューポートのジオメトリを取得
    QRect vpRect = viewport()->geometry();

    // ヘッダーをビューポートの上のマージン領域に配置
    topHeaderLabel->setGeometry(vpRect.left(), 0, vpRect.width(), topHeaderLabel->height());

    if (contentWidget) {
        // コンテンツウィジェットのサイズと位置を更新
        // マージンを考慮したビューポートのクライアント領域
        QRect contentRect = viewport()->rect();
        contentWidget->resize(contentRect.size());

        // スクロール位置に基づいてコンテンツウィジェットを移動
        int hOffset = horizontalScrollBar()->value();
        int vOffset = verticalScrollBar()->value();
        contentWidget->move(-hOffset, -vOffset); // スクロールバーの値の分だけ逆方向に移動
    }

    // スクロールバーの範囲を更新
    if (contentWidget) {
        QSize contentSize = contentWidget->sizeHint();
        QSize viewportSize = viewport()->size();

        horizontalScrollBar()->setRange(0, qMax(0, contentSize.width() - viewportSize.width()));
        verticalScrollBar()->setRange(0, qMax(0, contentSize.height() - viewportSize.height()));
    }
}

void MyScrollArea::scrollContentsBy(int dx, int dy)
{
    // スクロールバーの値に基づいてコンテンツを移動
    if (contentWidget) {
        contentWidget->move(contentWidget->x() + dx, contentWidget->y() + dy);
    }
}

main.cpp

#include <QApplication>
#include <QMainWindow>
#include <QPlainTextEdit> // スクロールさせるコンテンツとして使用
#include <QVBoxLayout>
#include "my_scroll_area.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QMainWindow window;
    window.setWindowTitle("Custom Scroll Area with Header");
    window.setGeometry(100, 100, 600, 400);

    MyScrollArea *scrollArea = new MyScrollArea();

    // スクロールさせる長いテキストコンテンツを作成
    QPlainTextEdit *plainTextEdit = new QPlainTextEdit();
    plainTextEdit->setPlainText(
        "これは長いテキストコンテンツです。\n"
        "スクロールエリアの動作を確認するために、たくさんの行と列を持っています。\n"
        ".......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................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.........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................QAbstractScrollArea::viewportMargins() は、Qtのアプリケーションにおいて、**QAbstractScrollArea クラスとそのサブクラス（QScrollArea など）のビューポート（実際に内容が表示される領域）の周りの余白**を管理するための重要な機能です。この余白は、スクロールされるコンテンツ自体とは別に、固定されたウィジェット（例：ヘッダー）を配置するために利用されます。

以下に、この機能に関連する代表的なコード例と、それぞれのポイントを日本語で説明します。

---

### 例1: `QAbstractScrollArea` を継承したカスタムウィジェットに固定ヘッダーを配置する

この例は、`QAbstractScrollArea` を継承して、ビューポートの上部に固定されたヘッダーを表示するカスタムスクロールエリアを作成する方法を示します。

**ポイント:**

* `QAbstractScrollArea` を直接継承します。`QScrollArea` とは異なり、ビューポート内のコンテンツの配置やスクロールバーの制御を開発者自身が行う必要があります。
* `setViewportMargins()` を使用して、ヘッダー分のマージンを上部に設定します。
* `resizeEvent()` をオーバーライドして、ウィジェットのサイズ変更時にヘッダーとビューポート内のコンテンツの位置とサイズを再計算・調整します。
* `scrollContentsBy()` をオーバーライドして、スクロールバーが動いたときにビューポート内のコンテンツを移動させます。

**コード:**

**`mycustomscrollarea.h`**

```cpp
#ifndef MYCUSTOMSCROLLAREA_H
#define MYCUSTOMSCROLLAREA_H

#include <QAbstractScrollArea>
#include <QLabel> // 固定ヘッダーとして使用
#include <QTextEdit> // スクロールするコンテンツとして使用
#include <QResizeEvent>
#include <QScrollBar>

class MyCustomScrollArea : public QAbstractScrollArea
{
    Q_OBJECT

public:
    explicit MyCustomScrollArea(QWidget *parent = nullptr);
    ~MyCustomScrollArea();

    // スクロールされるコンテンツを設定するメソッド
    void setContentWidget(QWidget *widget);

protected:
    // ウィジェットのサイズ変更イベント
    void resizeEvent(QResizeEvent *event) override;
    // スクロールバーの値が変更されたときにコンテンツをスクロール
    void scrollContentsBy(int dx, int dy) override;

private:
    QLabel *m_headerLabel; // ビューポート上部に固定表示するヘッダー
    QWidget *m_contentWidget; // スクロールされる実際のコンテンツ
};

#endif // MYCUSTOMSCROLLAREA_H

mycustomscrollarea.cpp

#include "mycustomscrollarea.h"
#include <QDebug>
#include <QApplication> // アプリケーション実行用

MyCustomScrollArea::MyCustomScrollArea(QWidget *parent)
    : QAbstractScrollArea(parent),
      m_contentWidget(nullptr)
{
    // 1. ヘッダーウィジェットの初期化とスタイル設定
    m_headerLabel = new QLabel("これはカスタムヘッダーです", this);
    m_headerLabel->setStyleSheet("background-color: #e0e0ff; border-bottom: 1px solid #a0a0ff; padding: 5px;");
    m_headerLabel->setAlignment(Qt::AlignCenter);
    m_headerLabel->setFixedHeight(40); // ヘッダーの高さを固定

    // 2. ビューポートのマージンを設定
    // 上部にヘッダーの高さ分、下部に少し余白を設ける
    // setViewportMargins(left, top, right, bottom)
    setViewportMargins(0, m_headerLabel->height(), 0, 10); // 下に10pxの余白を追加

    // 3. スクロールバーのポリシー設定
    setHorizontalScrollBarPolicy(Qt::ScrollBarAsNeeded);
    setVerticalScrollBarPolicy(Qt::ScrollBarAsNeeded);

    // 4. スクロールバーの値が変更されたときに、自身を更新するように接続
    // これにより、scrollContentsBy() が呼び出され、コンテンツが移動します。
    connect(horizontalScrollBar(), &QScrollBar::valueChanged, this, [this]{
        if (m_contentWidget) {
            scrollContentsBy(0, 0); // スクロールバーの値を反映させるために呼び出し
        }
    });
    connect(verticalScrollBar(), &QScrollBar::valueChanged, this, [this]{
        if (m_contentWidget) {
            scrollContentsBy(0, 0); // スクロールバーの値を反映させるために呼び出し
        }
    });
}

MyCustomScrollArea::~MyCustomScrollArea()
{
    // m_headerLabel と m_contentWidget は親に設定されているので、
    // 親ウィジェットが破棄されるときに自動的に破棄されます。
    // 明示的なdeleteは通常不要ですが、親設定がない場合は必要です。
}

void MyCustomScrollArea::setContentWidget(QWidget *widget)
{
    if (m_contentWidget) {
        m_contentWidget->setParent(nullptr); // 既存のコンテンツの親を解除
    }
    m_contentWidget = widget;
    if (m_contentWidget) {
        m_contentWidget->setParent(viewport()); // ビューポートの子として設定
        m_contentWidget->move(0, 0); // 初期位置
        m_contentWidget->show();
        viewport()->setMinimumSize(m_contentWidget->sizeHint()); // ビューポートの最小サイズをコンテンツに合わせる
        updateGeometry(); // レイアウト更新を促す
    }
}

void MyCustomScrollArea::resizeEvent(QResizeEvent *event)
{
    // QAbstractScrollAreaの親クラスのresizeEventを呼び出す
    // これにより、ビューポート自体のサイズが更新されます
    QAbstractScrollArea::resizeEvent(event);

    // ビューポートの現在のジオメトリを取得
    QRect vpRect = viewport()->geometry();

    // ヘッダーウィジェットをビューポートの上部マージン領域に配置
    m_headerLabel->setGeometry(vpRect.left(), 0, vpRect.width(), m_headerLabel->height());

    if (m_contentWidget) {
        // コンテンツウィジェットのサイズをビューポートのサイズに合わせる（もしコンテンツが小さい場合）
        // 大きすぎる場合はスクロールバーが対応する
        m_contentWidget->resize(qMax(vpRect.width(), m_contentWidget->sizeHint().width()),
                                qMax(vpRect.height(), m_contentWidget->sizeHint().height()));

        // スクロールバーの範囲を更新
        horizontalScrollBar()->setRange(0, qMax(0, m_contentWidget->width() - vpRect.width()));
        verticalScrollBar()->setRange(0, qMax(0, m_contentWidget->height() - vpRect.height()));
    }

    // スクロール位置に基づいてコンテンツウィジェットを移動
    scrollContentsBy(0, 0); // 値が変更されていなくても、現在のスクロール位置を反映させる
}

void MyCustomScrollArea::scrollContentsBy(int dx, int dy)
{
    if (m_contentWidget) {
        // スクロールバーの現在の値を取得
        int hOffset = horizontalScrollBar()->value();
        int vOffset = verticalScrollBar()->value();

        // コンテンツウィジェットをスクロール位置の逆方向に移動
        // これにより、ビューポート内の見える部分が適切に表示される
        m_contentWidget->move(-hOffset, -vOffset);
    }
}

#include <QApplication>
#include <QMainWindow>
#include <QTextEdit>
#include "mycustomscrollarea.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QMainWindow window;
    window.setWindowTitle("QAbstractScrollArea Viewport Margins Example");
    window.setGeometry(100, 100, 600, 400);

    MyCustomScrollArea *customScrollArea = new MyCustomScrollArea(&window);

    // 長いテキストを持つQTextEditをコンテンツとして設定
    QTextEdit *content = new QTextEdit();
    content->setPlainText(
        "これはスクロール可能なコンテンツの長いテキストです。\n"
        "たくさんの行があります。\n"
        ".......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................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---

QMargins QAbstractScrollArea::viewportMargins() が適さない、またはより柔軟なレイアウトが必要な場合に考えられる代替手段は以下の通りです。

QScrollArea と QWidget の組み合わせ

利点

• 保守性
  コードがシンプルで理解しやすいため、メンテナンスが容易です。
• Qtのレイアウトマネージャーの活用
  QVBoxLayout や QHBoxLayout などの強力なレイアウトシステムを使って、複雑なUIを簡単に構築できます。固定要素とスクロール要素のサイズ変更や配置が自動的に処理されます。
• 簡単
  QScrollArea::setWidget() を使うだけで、基本的なスクロール機能が実現できます。

欠点

• カスタムヘッダーの配置が難しい場合がある
  ヘッダーがスクロールエリアの端に「張り付く」ように見せたい場合、QScrollArea の親ウィジェットのレイアウトを工夫する必要があります。
• 固定要素がスクロールしない
  この方法では、固定要素（ヘッダーなど）は QScrollArea 自体の外部、または QScrollArea の setWidget() で設定されたウィジェットの内部に、スクロールとは関係なく固定されます。つまり、QScrollArea のスクロールバーが動いても、固定要素は動きません。

コード例 (概念)

// main.cpp またはメインウィンドウのコンストラクタ内で
#include <QApplication>
#include <QMainWindow>
#include <QScrollArea>
#include <QVBoxLayout>
#include <QLabel>
#include <QTextEdit>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QMainWindow window;
    window.setWindowTitle("QScrollArea Alternative");

    QWidget *centralWidget = new QWidget(&window);
    QVBoxLayout *mainLayout = new QVBoxLayout(centralWidget);
    mainLayout->setContentsMargins(0, 0, 0, 0); // 中央ウィジェットのマージンを0に

    // 1. 固定ヘッダーの配置
    QLabel *header = new QLabel("固定ヘッダー", centralWidget);
    header->setFixedHeight(40);
    header->setStyleSheet("background-color: lightgray; border-bottom: 1px solid darkgray;");
    header->setAlignment(Qt::AlignCenter);
    mainLayout->addWidget(header);

    // 2. スクロールエリアの設定
    QScrollArea *scrollArea = new QScrollArea(centralWidget);
    scrollArea->setWidgetResizable(true); // スクロールエリアがリサイズされたときに内部ウィジェットもリサイズ

    // 3. スクロールさせるコンテンツを作成
    QTextEdit *content = new QTextEdit();
    content->setPlainText("これはスクロールされるコンテンツです。\n"
                         "たくさんの行と長い行を持つテキストです。\n"
                         "これにより、スクロールバーが表示されるはずです。\n"
                         ".................................................\n"
                         "（以下、大量のテキストが続く）");
    content->setReadOnly(true); // 編集不可にする

    // 4. スクロールエリアにコンテンツを設定
    scrollArea->setWidget(content); // QScrollAreaは一つのウィジェットのみを管理

    // 5. メインレイアウトにスクロールエリアを追加
    mainLayout->addWidget(scrollArea);

    window.setCentralWidget(centralWidget);
    window.resize(400, 300);
    window.show();

    return a.exec();
}

QGraphicsView / QGraphicsScene を使用する

利点

• インタラクション
  アイテムごとのイベント処理やインタラクションが容易です。
• 効率的な描画
  大量のアイテムがあっても、QtのGraphics View Frameworkが効率的に描画を処理します。
• 高い柔軟性
  あらゆる種類のグラフィカル要素を自由に配置・描画できます。

欠点

• UI要素との統合
  QPushButton のような標準のQtウィジェットを直接 QGraphicsScene に追加するには、QGraphicsProxyWidget を使用する必要があります。
• 複雑性
  学習コストが高く、単純なUIにはオーバーキルになる可能性があります。

コード例 (概念)

// main.cpp またはメインウィンドウのコンストラクタ内で
#include <QApplication>
#include <QMainWindow>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsTextItem>
#include <QGraphicsRectItem>
#include <QVBoxLayout>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QMainWindow window;
    window.setWindowTitle("QGraphicsView Alternative");

    QGraphicsScene *scene = new QGraphicsScene();
    QGraphicsView *view = new QGraphicsView(scene, &window);
    view->setRenderHint(QPainter::Antialiasing); // アンチエイリアシング有効化

    // 1. スクロールされるコンテンツ（テキスト）
    QGraphicsTextItem *longText = new QGraphicsTextItem();
    longText->setHtml("<h1>これはスクロール可能なテキストコンテンツです</h1><p>たくさんの行と長い行を持つテキストです。<br>これにより、スクロールバーが表示されるはずです。<br>.................................................<br>（以下、大量のテキストが続く）</p>");
    longText->setTextWidth(400); // テキストの幅を固定
    scene->addItem(longText);

    // 2. 固定ヘッダー（QGraphicsRectItem と QGraphicsTextItem の組み合わせ）
    QGraphicsRectItem *headerBackground = new QGraphicsRectItem(0, 0, 400, 40); // シーン座標で作成
    headerBackground->setBrush(QBrush(QColor("#e0e0ff")));
    headerBackground->setZValue(100); // 手前に表示
    scene->addItem(headerBackground);

    QGraphicsTextItem *headerText = new QGraphicsTextItem("固定ヘッダー", headerBackground);
    headerText->setDefaultTextColor(Qt::blue);
    headerText->setFont(QFont("Arial", 12, QFont::Bold));
    headerText->setPos(10, 10); // ヘッダー背景に対する相対位置
    headerText->setZValue(101); // ヘッダー背景よりも手前に表示

    // ヘッダーをビューポートの上部に固定する（スクロールしないようにする）
    // QGraphicsViewのスクロールバーはシーンの範囲を基準に動くため、
    // 固定したいアイテムはビューポートの矩形に合わせて位置を調整する必要があります。
    // これはresizeEventやscrollContentsBy()に似たロジックをView側で実装するか、
    // QGraphicsViewのsetViewportUpdateMode()とsetSceneRect()を工夫する必要があります。
    // 例: headerBackground->setFlag(QGraphicsItem::ItemIgnoresTransformations);
    // これを設定すると、アイテムはビューの変換（スクロールやズーム）の影響を受けません。
    headerBackground->setFlag(QGraphicsItem::ItemIgnoresTransformations); // これが最も重要！

    window.setCentralWidget(view);
    window.resize(400, 300);
    window.show();

    return a.exec();
}

カスタムペインティングとスクロールバーの手動制御

利点

• 軽量
  複雑なフレームワークのオーバーヘッドがありません。
• 完全な制御
  描画とスクロールのロジックを完全にカスタマイズできます。

欠点

• 既存ウィジェットの描画が難しい
  QTextEdit のような複雑なウィジェットをこの方法でスクロールさせるのは非常に困難です。主にカスタムグラフィックスを描画する場合に適しています。
• 実装が複雑
  描画、スクロールバーの範囲設定、イベント処理などをすべて手動で行う必要があり、非常に手間がかかります。

// これは非常に複雑になるため、概念のみを示す
#include <QWidget>
#include <QPainter>
#include <QScrollBar>
#include <QVBoxLayout>

class MyCustomPaintedScrollArea : public QWidget
{
    Q_OBJECT

public:
    explicit MyCustomPaintedScrollArea(QWidget *parent = nullptr) : QWidget(parent)
    {
        // 垂直スクロールバーと水平スクロールバーをインスタンス化
        m_vScrollBar = new QScrollBar(Qt::Vertical, this);
        m_hScrollBar = new QScrollBar(Qt::Horizontal, this);

        // レイアウトにスクロールバーと描画領域を配置
        QVBoxLayout *mainLayout = new QVBoxLayout(this);
        QHBoxLayout *contentLayout = new QHBoxLayout();

        contentLayout->addWidget(this); // 自身（描画領域）を配置
        contentLayout->addWidget(m_vScrollBar);

        mainLayout->addLayout(contentLayout);
        mainLayout->addWidget(m_hScrollBar);

        // スクロールバーの値変更シグナルと描画更新スロットを接続
        connect(m_vScrollBar, &QScrollBar::valueChanged, this, [this](int value){
            m_vOffset = value;
            update(); // 再描画
        });
        connect(m_hScrollBar, &QScrollBar::valueChanged, this, [this](int value){
            m_hOffset = value;
            update(); // 再描画
        });

        // コンテンツの総サイズを設定するロジックが必要
        // m_vScrollBar->setRange(0, totalContentHeight - visibleHeight);
    }

protected:
    void paintEvent(QPaintEvent *event) override
    {
        QPainter painter(this);
        // 固定ヘッダーを描画
        painter.fillRect(0, 0, width(), 40, Qt::blue);
        painter.drawText(QRect(0, 0, width(), 40), Qt::AlignCenter, "カスタム描画ヘッダー");

        // スクロールされるコンテンツを描画
        // m_hOffset と m_vOffset を使って描画位置を調整
        // painter.drawText(x - m_hOffset, y - m_vOffset, "コンテンツ");
        // ... ここでコンテンツを描画 ...
    }

private:
    QScrollBar *m_vScrollBar;
    QScrollBar *m_hScrollBar;
    int m_hOffset = 0; // 水平スクロールオフセット
    int m_vOffset = 0; // 垂直スクロールオフセット
};

• Qtのウィジェットシステムを避け、ピクセル単位で完全に描画を制御したい場合
  カスタムペインティングとスクロールバーの手動制御ですが、これは最後の手段と考えるべきです。
• 高度なカスタム描画、ゲーム、または非常に多くのグラフィック要素
  QGraphicsView / QGraphicsScene が適しています。QGraphicsItem::ItemIgnoresTransformations フラグを使うことで、スクロールに追従しない固定要素を簡単に作成できます。
• 最も一般的なシナリオ（固定ヘッダーとスクロールするウィジェット）
  QScrollArea と QWidget の組み合わせが最も簡単で推奨されます。ヘッダーなどの固定要素は QScrollArea の外に配置し、QScrollArea の setWidget() にスクロールさせたいコンテンツ全体を設定します。