OmaPhoto 1.0.0: the Linux port of Compositor

A layered image editor for Linux and Omarchy, ported from Compositor
(github.com/robbietilton/Compositor), whose Swift source is its
specification. It reads and writes Compositor's .comp projects.

Packages for Ubuntu, Fedora and Arch come from the release workflow;
scripts/install/ holds one install script per distribution family,
and flake.nix serves NixOS.

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

src/UI/SliderSnap.cpp

@@ -0,0 +1,73 @@
+#include "UI/SliderSnap.h"
+#include <QApplication>
+#include <QMouseEvent>
+#include <QSlider>
+#include <QStyle>
+#include <QStyleOptionSlider>
+#include <algorithm>
+
+namespace {
+// Swift's snapValue: the value under a press on the track.
+class Snap : public QObject {
+public:
+    using QObject::QObject;
+
+protected:
+    bool eventFilter(QObject *watched, QEvent *event) override
+    {
+        auto *slider = qobject_cast<QSlider *>(watched);
+        if (!slider || event->type() != QEvent::MouseButtonPress)
+            return false;
+        const auto *press = static_cast<QMouseEvent *>(event);
+        // Qt refuses a press made with another button held.
+        if (press->buttons() != Qt::LeftButton || slider->orientation() != Qt::Horizontal || !slider->isEnabled()
+            || slider->maximum() <= slider->minimum())
+            return false;
+        // QSlider's own style option, which it keeps protected.
+        QStyleOptionSlider option;
+        option.initFrom(slider);
+        option.orientation = Qt::Horizontal;
+        option.state |= QStyle::State_Horizontal;
+        option.minimum = slider->minimum();
+        option.maximum = slider->maximum();
+        option.tickPosition = slider->tickPosition();
+        option.tickInterval = slider->tickInterval();
+        option.upsideDown = slider->invertedAppearance() != (option.direction == Qt::RightToLeft);
+        option.direction = Qt::LeftToRight;
+        option.sliderPosition = slider->sliderPosition();
+        option.sliderValue = slider->value();
+        option.singleStep = slider->singleStep();
+        option.pageStep = slider->pageStep();
+        const QRect groove = slider->style()->subControlRect(QStyle::CC_Slider, &option, QStyle::SC_SliderGroove, slider);
+        const QRect knob = slider->style()->subControlRect(QStyle::CC_Slider, &option, QStyle::SC_SliderHandle, slider);
+        const QPoint point = press->position().toPoint();
+        // The knob itself drags from where it is.
+        if (knob.contains(point) || groove.width() <= knob.width())
+            return false;
+        // Down first, as Swift's editing begins before the value.
+        slider->setSliderDown(true);
+        slider->setValue(QStyle::sliderValueFromPosition(slider->minimum(), slider->maximum(), point.x() - groove.x() - knob.width() / 2,
+                                                         groove.width() - knob.width(), option.upsideDown));
+        // A rounding owner may move the knob: press there.
+        option.sliderPosition = slider->sliderPosition();
+        option.sliderValue = slider->value();
+        const QRect moved = slider->style()->subControlRect(QStyle::CC_Slider, &option, QStyle::SC_SliderHandle, slider);
+        const QPoint inside(std::clamp(point.x(), moved.left(), moved.right()), std::clamp(point.y(), moved.top(), moved.bottom()));
+        const QPointF shift = inside - point;
+        QMouseEvent forwarded(QEvent::MouseButtonPress, inside, press->scenePosition() + shift, press->globalPosition() + shift, Qt::LeftButton,
+                              Qt::LeftButton, press->modifiers());
+        QCoreApplication::sendEvent(slider, &forwarded);
+        return true;
+    }
+};
+}
+
+void SliderSnap::install()
+{
+    // Once, as Swift's lazy static swaps the method once.
+    static const bool installed = [] {
+        qApp->installEventFilter(new Snap(qApp));
+        return true;
+    }();
+    (void)installed;
+}