Do not update source positions once initizlized

Author: romanowski

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1399,7 +1399,7 @@ class TreeUnpickler(reader: TastyReader,
       if (path.nonEmpty) {
         val sourceFile = ctx.getSource(path)
         posUnpicklerOpt match
-          case Some(posUnpickler) =>
+          case Some(posUnpickler) if !sourceFile.initizlized =>
             sourceFile.setLineIndicesFromLineSizes(posUnpickler.lineSizes)
           case _ =>
         pickling.println(i"source change at $addr: $path")

compiler/src/dotty/tools/dotc/util/SourceFile.scala

@@ -144,6 +144,9 @@ class SourceFile(val file: AbstractFile, computeContent: => Array[Char]) extends
     if lineIndicesCache eq null then
       lineIndicesCache = calculateLineIndicesFromContents()
     lineIndicesCache
+
+  def initizlized = lineIndicesCache != null
+
   def setLineIndicesFromLineSizes(sizes: Array[Int]): Unit =
     val lines = sizes.length
     val indices = new Array[Int](lines + 1)