perf: skip redundant progress writes when position is unchanged (#2436)

Co-authored-by: Erica Jensen <erica@mailershaven.com>
Co-authored-by: Uri Tauber <uritaube@gmail.com>

src/activities/reader/EpubReaderActivity.cpp

@@ -1207,7 +1207,17 @@ void EpubReaderActivity::render(RenderLock&& lock) {
     renderContents(std::move(p), orientedMarginTop, orientedMarginRight, orientedMarginBottom, orientedMarginLeft);
     LOG_DBG("ERS", "Rendered page in %dms", millis() - start);
   }
-  saveProgress(currentSpineIndex, section->currentPage, section->estimatedTotalPages());
+  // Only persist when the position actually changed. render() also runs on menu,
+  // bookmark and screenshot re-renders, and writeAtomic is several FAT ops for 6 bytes.
+  // Every real page turn changes currentPage, so progress durability is unaffected.
+  if (currentSpineIndex != lastSavedSpineIndex || section->currentPage != lastSavedPage ||
+      section->pageCount != lastSavedPageCount) {
+    if (saveProgress(currentSpineIndex, section->currentPage, section->estimatedTotalPages())) {
+      lastSavedSpineIndex = currentSpineIndex;
+      lastSavedPage = section->currentPage;
+      lastSavedPageCount = section->estimatedTotalPages();
+    }
+  }
 
   showPendingSyncSaveError();