Adjust EpubNavigatorFragment.firstVisibleElementLocator() strategy (#331)

Author: mickael-menu

CHANGELOG.md

@@ -15,6 +15,11 @@ All notable changes to this project will be documented in this file. Take a look
     * The native font size strategy introduced in 2.3.0 uses the Android web view's [`WebSettings.textZoom`](https://developer.android.com/reference/android/webkit/WebSettings#setTextZoom(int)) property to adjust the font size. 2.2.0 was using Readium CSS's [`--USER__fontSize` variable](https://readium.org/readium-css/docs/CSS12-user_prefs.html#font-size).
     * `WebSettings.textZoom` will work with more publications than `--USER__fontSize`, even the ones poorly authored. However the page width is not adjusted when changing the font size to keep the optimal line length.
 
+#### Changed
+
+* `EpubNavigatorFragment.firstVisibleElementLocator()` now returns the first *block* element that is visible on the screen, even if it starts on previous pages.
+    * This is used to make sure the user will not miss any context when restoring a TTS session in the middle of a resource.
+
 ## [2.3.0]
 
 ### Added

readium/navigator/src/main/assets/_scripts/src/dom.js

@@ -4,15 +4,11 @@
 //  available in the top-level LICENSE file of the project.
 //
 
-import { isScrollModeEnabled, pageWidth } from "./utils";
+import { isScrollModeEnabled } from "./utils";
 import { getCssSelector } from "css-selector-generator";
 
 export function findFirstVisibleLocator() {
   const element = findElement(document.body);
-  if (!element) {
-    return undefined;
-  }
-
   return {
     href: "#",
     type: "application/xhtml+xml",
@@ -26,59 +22,38 @@ export function findFirstVisibleLocator() {
 }
 
 function findElement(rootElement) {
-  var foundElement = undefined;
-  for (var i = rootElement.children.length - 1; i >= 0; i--) {
+  for (var i = 0; i < rootElement.children.length; i++) {
     const child = rootElement.children[i];
-    const position = elementRelativePosition(child, undefined);
-    if (position == 0) {
-      if (!shouldIgnoreElement(child)) {
-        foundElement = child;
-      }
-    } else if (position < 0) {
-      if (!foundElement) {
-        foundElement = child;
-      }
-      break;
+    if (!shouldIgnoreElement(child) && isElementVisible(child)) {
+      return findElement(child);
     }
   }
-
-  if (foundElement) {
-    return findElement(foundElement);
-  }
   return rootElement;
 }
 
-// See computeVisibility_() in r2-navigator-js
-function elementRelativePosition(element, domRect /* nullable */) {
+function isElementVisible(element) {
   if (readium.isFixedLayout) return true;
 
   if (element === document.body || element === document.documentElement) {
-    return -1;
+    return true;
   }
   if (!document || !document.documentElement || !document.body) {
-    return 1;
+    return false;
   }
 
-  const rect = domRect || element.getBoundingClientRect();
-
+  const rect = element.getBoundingClientRect();
   if (isScrollModeEnabled()) {
-    return rect.top >= 0 && rect.top <= document.documentElement.clientHeight;
+    return rect.bottom > 0 && rect.top < window.innerHeight;
   } else {
-    if (rect.left >= pageWidth) {
-      return 1;
-    } else if (rect.left >= 0) {
-      return 0;
-    } else {
-      return -1;
-    }
+    return rect.right > 0 && rect.left < window.innerWidth;
   }
 }
 
 function shouldIgnoreElement(element) {
   const elStyle = getComputedStyle(element);
   if (elStyle) {
     const display = elStyle.getPropertyValue("display");
-    if (display === "none") {
+    if (display != "block") {
       return true;
     }
     // Cannot be relied upon, because web browser engine reports invisible when out of view in

readium/navigator/src/main/assets/readium/scripts/readium-fixed.js

@@ -903,7 +903,8 @@ class EpubNavigatorFragment internal constructor(
     )
 
     /**
-     * Returns the [Locator] to the first HTML element that begins on the current screen.
+     * Returns the [Locator] to the first HTML *block* element that is visible on the screen, even
+     * if it begins on previous screen pages.
      */
     @ExperimentalReadiumApi
     override suspend fun firstVisibleElementLocator(): Locator? {