Let the mouse wheel scroll past spin buttons that do not have the focus

A GtkSpinButton grabs the focus and spins its value on any wheel turn over
it, so scrolling through the params stopped at the first spin under the
pointer and changed it. A capture-phase scroll controller now scrolls the
enclosing view instead, unless the spin already has the focus.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/gui/window.cpp

@@ -43,6 +43,40 @@ Gtk::Label* sectionLabel(const char* text) {
     return label;
 }
 
+// A GtkSpinButton takes every wheel turn over it: it grabs the focus and
+// spins its value, so scrolling down a column of params stops dead at the
+// first spin under the pointer and quietly changes it. This hands the wheel to
+// the view instead unless the spin already has the focus -- the user clicked
+// into it and means to adjust it. It runs in the capture phase, ahead of the
+// spin's own controller, and so has to scroll the enclosing ScrolledWindow
+// itself: stopping the event there stops it for the window as well.
+void scrollPastUnlessFocused(Gtk::SpinButton& spin) {
+    auto scroll = Gtk::EventControllerScroll::create();
+    scroll->set_propagation_phase(Gtk::PropagationPhase::CAPTURE);
+    scroll->set_flags(Gtk::EventControllerScroll::Flags::VERTICAL);
+    auto* controller = scroll.get();
+    scroll->signal_scroll().connect(
+        [&spin, controller](double, double dy) {
+            if ((spin.get_state_flags() & Gtk::StateFlags::FOCUS_WITHIN) == Gtk::StateFlags::FOCUS_WITHIN) {
+                return false;
+            }
+            auto* scroller = spin.get_ancestor(GTK_TYPE_SCROLLED_WINDOW);
+            if (scroller == nullptr) return true;
+            auto adjustment = static_cast<Gtk::ScrolledWindow*>(scroller)->get_vadjustment();
+            // The step a ScrolledWindow itself takes for one notch of a wheel;
+            // a touchpad already reports its deltas in pixels.
+            double step = dy;
+            if (controller->get_unit() == Gdk::ScrollUnit::WHEEL) {
+                step *= std::pow(adjustment->get_page_size(), 2.0 / 3.0);
+            }
+            adjustment->set_value(std::clamp(adjustment->get_value() + step, adjustment->get_lower(),
+                                             adjustment->get_upper() - adjustment->get_page_size()));
+            return true;
+        },
+        false);
+    spin.add_controller(scroll);
+}
+
 }  // namespace
 
 EffectorWindow::EffectorWindow(const Options& initial) {
@@ -281,6 +315,7 @@ Gtk::Widget& EffectorWindow::buildCanvasSection() {
     dpiSpin_.set_value(kDefaultDpi);
     for (auto* spin : {&widthSpin_, &heightSpin_, &dpiSpin_}) {
         spin->set_hexpand(true);
+        scrollPastUnlessFocused(*spin);
     }
     for (auto* spin : {&widthSpin_, &heightSpin_}) {
         spin->signal_value_changed().connect([this] {
@@ -477,6 +512,7 @@ void EffectorWindow::buildParamWidgets() {
                                     : paramDouble(values, spec.key, spec.defaultValue));
                 spin->signal_value_changed().connect(
                     sigc::mem_fun(*this, &EffectorWindow::onParamChanged));
+                scrollPastUnlessFocused(*spin);
                 widget = std::move(spin);
                 break;
             }