Reduce page increment of GTK wxSpinButton for narrow range

Gradually reduce the page increment in the case of a narrow range for
convenience and to limit possible EVT_SPIN_UP/EVT_SPIN_DOWN ambiguity
when wrapping is on.

Closes https://github.com/wxWidgets/wxWidgets/pull/1900

src/gtk/spinbutt.cpp

@@ -186,6 +186,26 @@ void wxSpinButton::SetRange(int minVal, int maxVal)
     GtkDisableEvents();
     gtk_spin_button_set_range((GtkSpinButton*)m_widget, minVal, maxVal);
     m_pos = int(gtk_spin_button_get_value((GtkSpinButton*)m_widget));
+
+    // Use smaller page increment in the case of a narrow range for convenience
+    // and to limit possible up/down ambiguity in gtk_value_changed() when
+    // wrapping is on (The maximal page increment of 10 is consistent with the
+    // default page increment set by gtk_spin_button_new_with_range(0, 100, 1)
+    // in wxSpinButton::Create().)
+    const int range = maxVal - minVal;
+    int pageInc;
+    if ( range < 10 )
+        pageInc = 1;
+    else if ( range < 20 )
+        pageInc = 2;
+    else if ( range < 50 )
+        pageInc = 5;
+    else
+        pageInc = 10;
+
+    GtkAdjustment* adj = gtk_spin_button_get_adjustment((GtkSpinButton*)m_widget);
+    gtk_adjustment_set_page_increment(adj, pageInc);
+
     GtkEnableEvents();
 }