Prevent page deletion on save

main.go

@@ -426,9 +426,14 @@ func (h *handler) handlePost(w http.ResponseWriter, r *http.Request, urlPath, fs
 		}
 		rawMD, _ := os.ReadFile(indexPath)
 		sections := splitSections(rawMD)
-		if sectionIndex < len(sections) {
+		// Out of range means the file changed under the editor (or the index
-			sections[sectionIndex] = []byte(content)
+		// never matched it). Writing back the untouched file would swallow the
+		// edit silently, so refuse and keep the editor's content in the browser.
+		if sectionIndex >= len(sections) {
+			http.Error(w, "section no longer exists — the page changed since you opened the editor", http.StatusConflict)
+			return
 		}
+		sections[sectionIndex] = []byte(content)
 		content = string(joinSections(sections))
 		// Section index ≥ 1 is a heading-anchored section. Redirect to its
 		// anchor so the user lands on the section they just saved, even if
@@ -441,12 +446,15 @@ func (h *handler) handlePost(w http.ResponseWriter, r *http.Request, urlPath, fs
 		}
 	}
 
+	// A save must never remove a page. An empty POST — a truncated mobile
+	// request, a lost `section` field, an editor that came up blank — is
+	// indistinguishable from "clear this page", and deleting index.md on that
+	// signal loses the whole file even though the user only edited one section.
+	// Removing a page is the explicit ?delete action's job (moves.go).
 	if strings.TrimSpace(content) == "" {
-		if err := os.Remove(indexPath); err != nil && !os.IsNotExist(err) {
+		http.Error(w, "refusing to save empty content — use DELETE to remove this page", http.StatusBadRequest)
-			http.Error(w, "delete failed: "+err.Error(), http.StatusInternalServerError)
 		return
 	}
-	} else {
 	// Stat first so we know whether MkdirAll actually created the folder
 	// — if it did, the search index needs a new entry.
 	_, statErr := os.Stat(fsPath)
@@ -464,7 +472,6 @@ func (h *handler) handlePost(w http.ResponseWriter, r *http.Request, urlPath, fs
 			folderIndexAdd(filepath.ToSlash(rel))
 		}
 	}
-	}
 	// The editor saves via fetch so the save and its result share one history
 	// entry (see assets/history-nav.js). Hand it the target instead of a 303:
 	// the browser would follow the redirect into a second entry, and fetch