Fix scrollback: burst-proof stick-to-bottom, cap-proof layout wait, serialized buffer updates, atomic history saves

src/main/kotlin/data/MessageHistoryRepository.kt

@@ -3,6 +3,9 @@ package data
 import org.json.JSONArray
 import org.json.JSONObject
 import java.io.File
+import java.nio.file.AtomicMoveNotSupportedException
+import java.nio.file.Files
+import java.nio.file.StandardCopyOption
 
 /**
  * Persists each buffer's scrollback to a JSON file under rootDir/history/, so messages
@@ -25,14 +28,31 @@ class MessageHistoryRepository(private val rootDir: File) {
             val arr = JSONArray(file.readText())
             (0 until arr.length()).map { i -> fromJson(arr.getJSONObject(i)) }
         } catch (e: Exception) {
+            // Rename the bad file aside rather than silently returning empty - returning
+            // empty here looks to the caller exactly like "no history yet", and the next
+            // debounced save would then happily overwrite - and permanently lose - whatever
+            // was in the corrupt file.
+            file.renameTo(File(file.parentFile, "${file.name}.corrupt"))
             emptyList()
         }
     }
 
+    // @Synchronized because flushAllSaves (on shutdown) calls this directly for every
+    // buffer while a debounced save for the same key can still be in flight on its own
+    // coroutine - without this, the two writes race and whichever's atomic move lands
+    // second wins, which isn't necessarily the one with the more complete message list.
+    @Synchronized
     fun saveMessages(key: BufferKey, messages: List<IrcMessage>) {
         val arr = JSONArray()
         messages.takeLast(500).forEach { arr.put(toJson(it)) }
-        fileFor(key).writeText(arr.toString())
+        val file = fileFor(key)
+        val tmp = File(file.parentFile, "${file.name}.tmp")
+        tmp.writeText(arr.toString())
+        try {
+            Files.move(tmp.toPath(), file.toPath(), StandardCopyOption.REPLACE_EXISTING, StandardCopyOption.ATOMIC_MOVE)
+        } catch (e: AtomicMoveNotSupportedException) {
+            Files.move(tmp.toPath(), file.toPath(), StandardCopyOption.REPLACE_EXISTING)
+        }
     }
 
     private fun toJson(m: IrcMessage): JSONObject = JSONObject().apply {