prwevent scroll-driven zoom in pianoroll from exceeding a reasonable level

gtk2_ardour/editing_context.cc

@@ -2498,6 +2498,13 @@ EditingContext::reset_y_origin (double y)
 void
 EditingContext::reset_zoom (samplecnt_t spp)
 {
+	std::pair<timepos_t, timepos_t> ext = max_zoom_extent();
+	samplecnt_t max_extents_pp = (ext.second.samples() - ext.first.samples())  / _visible_canvas_width;
+
+	if (spp > max_extents_pp) {
+		spp = max_extents_pp;
+	}
+
 	if (spp == samples_per_pixel) {
 		return;
 	}