layoutCacheBytes

How much laid-out text the book keeps in memory, as an estimate in bytes.

Layout keeps one glyph object per character, so a whole novel costs more than an Android app's heap allows (about 165 bytes per character on the JVM). The chapters the reader has not used for the longest drop their pages first and lay out again on their next use, about a tenth of a second per chapter. Page counts, anchors and bookmarks survive the drop, so navigation never waits. One chapter always stays, so a book that is a single spine document keeps that document whole. 0 keeps only the chapter in use.