added css class for darker stroke on crossovers within the same helix

lib/src/constants.dart

@@ -430,6 +430,7 @@ const css_selector_domain = 'domain-line';
 const css_selector_base_pair_line = 'base-pair-line';
 const css_selector_extension = 'extension-line';
 const css_selector_crossover = 'crossover-curve';
+const css_selector_crossover_same_helix = 'crossover-curve-same-helix';
 const css_selector_loopout = 'loopout-curve';
 const css_selector_end_5p_strand = 'five-prime-end-first-substrand';
 const css_selector_end_3p_strand = 'three-prime-end-last-substrand';

lib/src/view/design_main_strand_crossover.dart

@@ -69,6 +69,13 @@ class DesignMainStrandCrossoverComponent
       classname += ' ' + constants.css_selector_scaffold;
     }
 
+    // Want to display crossovers within a helix a little darker since it's hard to see them;
+    // This is for "crossovers" that really are just buffered space between domains on the same helix.
+    if (props.prev_domain.helix == props.next_domain.helix &&
+        props.prev_domain.forward == props.next_domain.forward) {
+      classname += ' ' + constants.css_selector_crossover_same_helix;
+    }
+
     var prev_group = props.helices[props.prev_domain.helix].group;
     var next_group = props.helices[props.next_domain.helix].group;
     bool within_group = prev_group == next_group;

web/scadnano-styles.css

@@ -742,6 +742,10 @@ label + select {
     fill: none;
 }
 
+.crossover-curve-same-helix {
+    stroke-opacity: 0.55;
+}
+
 .potential-vertical-crossover-curve {
     stroke-width: 4;
     fill: none;