Use Kate's native diff viewer, add per-file and full changeset diff

- Integrate with libkateprivate's Utils::showDiff() to use Kate's built-in
  diff widget (respects user's Unified/SideBySide/Raw style preference)
- Add bundled kate-compat headers for DiffParams and Utils::showDiff decl
- CMakeLists.txt auto-detects libkateprivate, falls back to plain text diff
- Add 'Diff selected' button: shows diff for selected files with
  stage/unstage/discard actions available in the diff viewer
- Add 'Diff All' button: shows git diff HEAD for full changeset review
- Staged diffs show unstage action, unstaged diffs show stage+discard
- ReloadOnShow flag keeps diff views fresh after interactive operations

CMakeLists.txt

@@ -8,7 +8,7 @@ set(CMAKE_AUTOMOC ON)
 find_package(ECM 5.80 REQUIRED NO_MODULE)
 set(CMAKE_MODULE_PATH ${ECM_MODULE_PATH})
 
-# Probe for KF6 before including KDE macros (which need QT_MAJOR_VERSION)
+# --- Detect Qt6/KF6 or fall back to Qt5/KF5 ---
 find_package(KF6TextEditor QUIET CONFIG)
 if(KF6TextEditor_FOUND)
     set(QT_MAJOR_VERSION 6)
@@ -47,6 +47,18 @@ else()
     message(STATUS "Building Git++ with KF5/Qt5")
 endif()
 
+# --- Find libkateprivate for diff widget support ---
+# The library may only exist as a versioned .so (no dev symlink)
+file(GLOB _KATEPRIVATE_CANDIDATES "/usr/lib64/libkateprivate.so*" "/usr/lib/libkateprivate.so*")
+if(_KATEPRIVATE_CANDIDATES)
+    list(GET _KATEPRIVATE_CANDIDATES 0 KATEPRIVATE_LIB)
+    message(STATUS "Found libkateprivate: ${KATEPRIVATE_LIB}")
+    add_definitions(-DHAVE_KATE_DIFF)
+else()
+    set(KATEPRIVATE_LIB "")
+    message(STATUS "libkateprivate not found — diff will open as plain text")
+endif()
+
 kcoreaddons_add_plugin(gitplusplus
     SOURCES
         gitplusplus.cpp
@@ -55,5 +67,9 @@ kcoreaddons_add_plugin(gitplusplus
 )
 
 target_link_libraries(gitplusplus ${KF_LINK_LIBS})
-
 target_include_directories(gitplusplus PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
+
+if(KATEPRIVATE_LIB)
+    target_include_directories(gitplusplus PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/kate-compat)
+    target_link_libraries(gitplusplus ${KATEPRIVATE_LIB})
+endif()

gitplusplus.cpp

@@ -2,6 +2,10 @@
 #include "gitprocess.h"
 #include "gitstatusmodel.h"
 
+#ifdef HAVE_KATE_DIFF
+#include "ktexteditor_utils.h"
+#endif
+
 #include <KPluginFactory>
 #include <KLocalizedString>
 #include <ktexteditor_version.h>
@@ -139,6 +143,8 @@ GitPlusPlusView::GitPlusPlusView(GitPlusPlusPlugin *plugin, KTextEditor::MainWin
     m_stageBtn = makeToolButton(QStringLiteral("list-add"), QStringLiteral("Stage"));
     m_unstageBtn = makeToolButton(QStringLiteral("list-remove"), QStringLiteral("Unstage"));
     m_discardBtn = makeToolButton(QStringLiteral("edit-delete"), QStringLiteral("Discard"));
+    auto *diffBtn = makeToolButton(QStringLiteral("vcs-diff"), QStringLiteral("Diff selected (or all)"));
+    auto *diffAllBtn = makeToolButton(QStringLiteral("view-split-left-right"), QStringLiteral("Diff entire changeset"));
     m_commitBtn = makeToolButton(QStringLiteral("vcs-commit"), QStringLiteral("Commit"));
     m_pushBtn = makeToolButton(QStringLiteral("vcs-push"), QStringLiteral("Push"));
     m_pullBtn = makeToolButton(QStringLiteral("vcs-pull"), QStringLiteral("Pull"));
@@ -150,6 +156,8 @@ GitPlusPlusView::GitPlusPlusView(GitPlusPlusPlugin *plugin, KTextEditor::MainWin
     toolbar->addWidget(m_stageBtn);
     toolbar->addWidget(m_unstageBtn);
     toolbar->addWidget(m_discardBtn);
+    toolbar->addWidget(diffBtn);
+    toolbar->addWidget(diffAllBtn);
     toolbar->addWidget(m_commitBtn);
     toolbar->addWidget(m_pushBtn);
     toolbar->addWidget(m_pullBtn);
@@ -317,6 +325,8 @@ GitPlusPlusView::GitPlusPlusView(GitPlusPlusPlugin *plugin, KTextEditor::MainWin
     connect(m_stageBtn, &QToolButton::clicked, this, &GitPlusPlusView::stageSelected);
     connect(m_unstageBtn, &QToolButton::clicked, this, &GitPlusPlusView::unstageSelected);
     connect(m_discardBtn, &QToolButton::clicked, this, &GitPlusPlusView::discardSelected);
+    connect(diffBtn, &QToolButton::clicked, this, &GitPlusPlusView::diffSelected);
+    connect(diffAllBtn, &QToolButton::clicked, this, &GitPlusPlusView::diffAll);
     connect(m_commitBtn, &QToolButton::clicked, this, &GitPlusPlusView::commitChanges);
     connect(m_pushBtn, &QToolButton::clicked, this, &GitPlusPlusView::push);
     connect(m_pullBtn, &QToolButton::clicked, this, &GitPlusPlusView::pull);
@@ -467,6 +477,28 @@ void GitPlusPlusView::setMessage(const QString &text, bool isError)
     m_statusLabel->setStyleSheet(isError ? QStringLiteral("color: red;") : QStringLiteral("color: green;"));
 }
 
+void GitPlusPlusView::showDiffOutput(const QByteArray &diff, const QString &title, const QStringList &gitArgs, int extraFlags)
+{
+#ifdef HAVE_KATE_DIFF
+    DiffParams params;
+    params.tabTitle = title;
+    params.workingDir = m_gitDir;
+    params.arguments = gitArgs;
+    params.flags = DiffParams::Flag(DiffParams::ShowFileName | extraFlags);
+    params.updateStatusCallback = [this]() { updateStatus(); };
+    Utils::showDiff(diff, params, m_mainWindow);
+#else
+    Q_UNUSED(title);
+    Q_UNUSED(gitArgs);
+    Q_UNUSED(extraFlags);
+    auto *view = m_mainWindow->openUrl(QUrl());
+    if (view) {
+        view->document()->setText(QString::fromUtf8(diff));
+        view->document()->setHighlightingMode(QStringLiteral("Diff"));
+    }
+#endif
+}
+
 void GitPlusPlusView::refreshAll()
 {
     updateStatus();
@@ -872,6 +904,147 @@ void GitPlusPlusView::discardSelected()
                  QStringLiteral("Failed to discard"));
 }
 
+void GitPlusPlusView::diffSelected()
+{
+    // Get all selected file entries (from any section)
+    QStringList stagedFiles;
+    QStringList unstagedFiles;
+    const auto indexes = m_treeView->selectionModel()->selectedIndexes();
+    for (const auto &proxyIdx : indexes) {
+        QModelIndex idx = m_proxy->mapToSource(proxyIdx);
+        if (idx.internalId() == 0xFFFFFFFF)
+            continue; // skip top-level category nodes
+        int nodeType = int(idx.internalId());
+        QString file = idx.data(GitStatusModel::FilePathRole).toString();
+        if (file.isEmpty())
+            continue;
+        if (nodeType == GitStatusModel::NodeStaged)
+            stagedFiles.append(file);
+        else if (nodeType == GitStatusModel::NodeChanged)
+            unstagedFiles.append(file);
+        // Untracked files: show as new file diff
+        else if (nodeType == GitStatusModel::NodeUntracked)
+            unstagedFiles.append(file);
+    }
+
+    // If nothing selected, fall through to diffAll
+    if (stagedFiles.isEmpty() && unstagedFiles.isEmpty()) {
+        diffAll();
+        return;
+    }
+
+    // Show staged diff for selected staged files
+    if (!stagedFiles.isEmpty()) {
+        QStringList args = {QStringLiteral("diff"), QStringLiteral("--cached"), QStringLiteral("--")};
+        args.append(stagedFiles);
+        auto *proc = gitProcess(args);
+        if (!proc) return;
+        connect(proc, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
+                this, [this, proc, stagedFiles, args](int exitCode, QProcess::ExitStatus) {
+            if (exitCode == 0) {
+                QByteArray output = proc->readAllStandardOutput();
+                if (!output.trimmed().isEmpty()) {
+                    QString title = stagedFiles.size() == 1
+                        ? QStringLiteral("Staged: %1").arg(stagedFiles.first())
+                        : QStringLiteral("Staged changes (%1 files)").arg(stagedFiles.size());
+#ifdef HAVE_KATE_DIFF
+                    showDiffOutput(output, title, args,
+                                   DiffParams::ShowUnstage | DiffParams::ReloadOnShow);
+#else
+                    showDiffOutput(output, title, args);
+#endif
+                } else {
+                    setMessage(QStringLiteral("No staged diff for selection"));
+                }
+            }
+            proc->deleteLater();
+        });
+        proc->start();
+    }
+
+    // Show unstaged diff for selected changed/untracked files
+    if (!unstagedFiles.isEmpty()) {
+        QStringList args = {QStringLiteral("diff"), QStringLiteral("--")};
+        args.append(unstagedFiles);
+        auto *proc = gitProcess(args);
+        if (!proc) return;
+        connect(proc, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
+                this, [this, proc, unstagedFiles, args](int exitCode, QProcess::ExitStatus) {
+            if (exitCode == 0) {
+                QByteArray output = proc->readAllStandardOutput();
+                if (!output.trimmed().isEmpty()) {
+                    QString title = unstagedFiles.size() == 1
+                        ? QStringLiteral("Unstaged: %1").arg(unstagedFiles.first())
+                        : QStringLiteral("Unstaged changes (%1 files)").arg(unstagedFiles.size());
+#ifdef HAVE_KATE_DIFF
+                    showDiffOutput(output, title, args,
+                                   DiffParams::ShowStage | DiffParams::ShowDiscard | DiffParams::ReloadOnShow);
+#else
+                    showDiffOutput(output, title, args);
+#endif
+                } else {
+                    setMessage(QStringLiteral("No unstaged diff for selection"));
+                }
+            }
+            proc->deleteLater();
+        });
+        proc->start();
+    }
+}
+
+void GitPlusPlusView::diffAll()
+{
+    if (m_gitDir.isEmpty()) {
+        setMessage(QStringLiteral("No git directory found"), true);
+        return;
+    }
+
+    // Show combined diff: staged + unstaged in one view
+    // Use "git diff HEAD" which shows all changes (staged and unstaged) relative to HEAD
+    QStringList args = {QStringLiteral("diff"), QStringLiteral("HEAD")};
+    auto *proc = gitProcess(args);
+    if (!proc) return;
+    connect(proc, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
+            this, [this, proc, args](int exitCode, QProcess::ExitStatus) {
+        if (exitCode == 0) {
+            QByteArray output = proc->readAllStandardOutput();
+            if (output.trimmed().isEmpty()) {
+                setMessage(QStringLiteral("No changes to diff"));
+            } else {
+                showDiffOutput(output, QStringLiteral("All changes"), args);
+                setMessage(QStringLiteral("Showing all changes"));
+            }
+        } else {
+            // HEAD might not exist (initial commit) — fall back to staged + unstaged separately
+            QString err = QString::fromUtf8(proc->readAllStandardError()).trimmed();
+            if (err.contains(QStringLiteral("unknown revision"))) {
+                // No commits yet — show diff of staged (index vs empty)
+                auto *proc2 = gitProcess({QStringLiteral("diff"), QStringLiteral("--cached")});
+                if (proc2) {
+                    connect(proc2, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
+                            this, [this, proc2](int exitCode2, QProcess::ExitStatus) {
+                        if (exitCode2 == 0) {
+                            QByteArray out2 = proc2->readAllStandardOutput();
+                            if (!out2.trimmed().isEmpty()) {
+                                showDiffOutput(out2, QStringLiteral("All changes (initial commit)"),
+                                               {QStringLiteral("diff"), QStringLiteral("--cached")});
+                            } else {
+                                setMessage(QStringLiteral("No changes to diff"));
+                            }
+                        }
+                        proc2->deleteLater();
+                    });
+                    proc2->start();
+                }
+            } else {
+                setMessage(QStringLiteral("Diff failed: ") + err, true);
+            }
+        }
+        proc->deleteLater();
+    });
+    proc->start();
+}
+
 void GitPlusPlusView::commitChanges()
 {
     QString msg = m_commitMsgEdit->toPlainText().trimmed();
@@ -1046,16 +1219,12 @@ void GitPlusPlusView::compareBranch()
     connect(proc, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
             this, [this, proc, branch](int exitCode, QProcess::ExitStatus) {
         if (exitCode == 0) {
-            QString diff = QString::fromUtf8(proc->readAllStandardOutput());
-            if (diff.isEmpty()) {
+            QByteArray diff = proc->readAllStandardOutput();
+            if (diff.trimmed().isEmpty()) {
                 setMessage(QStringLiteral("No differences between %1 and %2").arg(m_currentBranch, branch));
             } else {
-                // Open diff in a new untitled document
-                auto *view = m_mainWindow->openUrl(QUrl());
-                if (view) {
-                    view->document()->setText(diff);
-                    view->document()->setHighlightingMode(QStringLiteral("Diff"));
-                }
+                showDiffOutput(diff, QStringLiteral("Diff: %1...%2").arg(m_currentBranch, branch),
+                               {QStringLiteral("diff"), m_currentBranch + QStringLiteral("...") + branch});
                 setMessage(QStringLiteral("Comparing %1...%2").arg(m_currentBranch, branch));
             }
         } else {
@@ -1292,15 +1461,12 @@ void GitPlusPlusView::stashDiff()
     connect(proc, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
             this, [this, proc, stash](int exitCode, QProcess::ExitStatus) {
         if (exitCode == 0) {
-            QString output = QString::fromUtf8(proc->readAllStandardOutput());
-            if (output.isEmpty()) {
+            QByteArray output = proc->readAllStandardOutput();
+            if (output.trimmed().isEmpty()) {
                 setMessage(QStringLiteral("Empty stash diff"));
             } else {
-                auto *view = m_mainWindow->openUrl(QUrl());
-                if (view) {
-                    view->document()->setText(output);
-                    view->document()->setHighlightingMode(QStringLiteral("Diff"));
-                }
+                showDiffOutput(output, QStringLiteral("Diff - %1").arg(stash),
+                               {QStringLiteral("stash"), QStringLiteral("show"), QStringLiteral("-p"), stash});
                 setMessage(QStringLiteral("Diff: ") + stash);
             }
         } else {
@@ -1344,12 +1510,9 @@ void GitPlusPlusView::openCommit()
     connect(proc, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
             this, [this, proc, ref](int exitCode, QProcess::ExitStatus) {
         if (exitCode == 0) {
-            QString output = QString::fromUtf8(proc->readAllStandardOutput());
-            auto *view = m_mainWindow->openUrl(QUrl());
-            if (view) {
-                view->document()->setText(output);
-                view->document()->setHighlightingMode(QStringLiteral("Diff"));
-            }
+            QByteArray output = proc->readAllStandardOutput();
+            showDiffOutput(output, QStringLiteral("Commit: %1").arg(ref),
+                           {QStringLiteral("show"), QStringLiteral("--stat"), QStringLiteral("--patch"), ref});
             setMessage(QStringLiteral("Showing commit: ") + ref);
         } else {
             QString err = QString::fromUtf8(proc->readAllStandardError()).trimmed();

gitplusplus.h

@@ -68,6 +68,8 @@ private Q_SLOTS:
     void unstageSelected();
     void discardSelected();
     void commitChanges();
+    void diffSelected();
+    void diffAll();
 
     // Remote
     void push();
@@ -111,6 +113,7 @@ private:
     void runSimpleGit(const QStringList &args, const QString &successMsg, const QString &errorMsg);
     void runRemoteGit(const QStringList &args, const QString &opName);
     void setMessage(const QString &text, bool isError = false);
+    void showDiffOutput(const QByteArray &diff, const QString &title, const QStringList &gitArgs = {}, int extraFlags = 0);
     void updateIndicators();
     void setRemoteBusy(bool busy, const QString &opName = {});
     void refreshAll();

kate-compat/diffparams.h

@@ -0,0 +1,44 @@
+/*
+    SPDX-FileCopyrightText: 2022 Waqar Ahmed <waqar.17a@gmail.com>
+    SPDX-License-Identifier: LGPL-2.0-or-later
+
+    Minimal copy of Kate's DiffParams for external plugin use.
+*/
+#pragma once
+
+#include <QMetaType>
+#include <QString>
+#include <QStringList>
+#include <functional>
+
+struct DiffParams {
+    enum Flag {
+        ShowStage = 1,
+        ShowUnstage = 2,
+        ShowDiscard = 4,
+        ShowFileName = 8,
+        ShowCommitInfo = 16,
+        ReloadOnShow = 32,
+        ShowFullContext = 64,
+        ShowEditLeftSide = 128,
+        ShowEditRightSide = 256
+    };
+    Q_DECLARE_FLAGS(Flags, Flag)
+
+    QString tabTitle;
+    QString srcFile;
+    QString destFile;
+    QString workingDir;
+    QStringList arguments;
+    Flags flags;
+
+    void clear()
+    {
+        tabTitle = srcFile = destFile = workingDir = QString();
+        arguments.clear();
+        flags = {};
+        updateStatusCallback = nullptr;
+    }
+
+    std::function<void()> updateStatusCallback;
+};

kate-compat/ktexteditor_utils.h

@@ -0,0 +1,16 @@
+/*
+    SPDX-FileCopyrightText: 2022 Waqar Ahmed <waqar.17a@gmail.com>
+    SPDX-License-Identifier: LGPL-2.0-or-later
+
+    Minimal declaration of Utils::showDiff for external plugin use.
+*/
+#pragma once
+
+#include "diffparams.h"
+#include <QByteArray>
+
+namespace KTextEditor { class MainWindow; }
+
+namespace Utils {
+void showDiff(const QByteArray &diff, const DiffParams &params, KTextEditor::MainWindow *mainWindow);
+}