feat: light a chapter when its heading crosses 60% of the viewport

src/journey/section-controller.ts

@@ -32,10 +32,17 @@ export interface ActivationLineInput {
   visualViewportHeight: number;
 }
 
+/**
+ * A chapter becomes current when its heading's top crosses this fraction of
+ * the visual viewport, measured from the top. At 60% the chapter lights up as
+ * its heading settles into the reading zone, not after it has been read.
+ */
+export const ACTIVATION_LINE_VH = 0.6;
+
 export const activationLineDocumentY = (input: ActivationLineInput): number =>
   input.scrollY +
   input.visualViewportOffsetTop +
-  input.visualViewportHeight * 0.35;
+  input.visualViewportHeight * ACTIVATION_LINE_VH;
 
 export interface DocumentEndInput {
   scrollY: number;