Completions should prepend, not replace as it is for Scala 2 (#18803)

By default, in Scala 2 completion prepend and only replace if there is
exact match. This PR unifies the experience between the Scala versions.
It seems more intuitive to work that way.

The change will be as follows (the order is: new logic, old logic and
scala 2 logic):


https://github.com/lampepfl/dotty/assets/48657087/c037c322-5613-4b95-a6e5-090b4e8827b6


In the future, it should be improved by changing implementation to use
`InsertReplaceEdit` instead of `TextEdit`. This will allow users to
decide which behaviour they prefer, but this has to be first implemented
in metals to properly handle the new logic, but for now the key is to
unify behaviours.
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#insertReplaceEdit

presentation-compiler/src/main/dotty/tools/pc/completions/CompletionPos.scala

@@ -29,10 +29,9 @@ case class CompletionPos(
 ):
 
   def sourcePos: SourcePosition = cursorPos.withSpan(Spans.Span(start, end))
+  def stripSuffixEditRange: l.Range = new l.Range(cursorPos.offsetToPos(start), cursorPos.offsetToPos(end))
+  def toEditRange: l.Range = cursorPos.withStart(start).withEnd(cursorPos.point).toLsp
 
-  def toEditRange: l.Range =
-    new l.Range(cursorPos.offsetToPos(start), cursorPos.offsetToPos(end))
-  end toEditRange
 end CompletionPos
 
 object CompletionPos:

presentation-compiler/src/main/dotty/tools/pc/completions/CompletionProvider.scala

@@ -151,10 +151,7 @@ class CompletionProvider(
       indexedContext: IndexedContext
   )(using ctx: Context): CompletionItem =
     val printer =
-      ShortenedTypePrinter(search, IncludeDefaultParam.ResolveLater)(using
-        indexedContext
-      )
-    val editRange = completionPos.toEditRange
+      ShortenedTypePrinter(search, IncludeDefaultParam.ResolveLater)(using indexedContext)
 
     // For overloaded signatures we get multiple symbols, so we need
     // to recalculate the description
@@ -165,24 +162,22 @@ class CompletionProvider(
     val ident = completion.insertText.getOrElse(completion.label)
 
     def mkItem(
-        insertText: String,
+        newText: String,
         additionalEdits: List[TextEdit] = Nil,
         range: Option[LspRange] = None
     ): CompletionItem =
-      val nameEdit = new TextEdit(
-        range.getOrElse(editRange),
-        insertText
-      )
+      val oldText = params.text.substring(completionPos.start, completionPos.end)
+      val editRange = if newText.startsWith(oldText) then completionPos.stripSuffixEditRange
+        else completionPos.toEditRange
+
+      val textEdit = new TextEdit(range.getOrElse(editRange), newText)
+
       val item = new CompletionItem(label)
       item.setSortText(f"${idx}%05d")
       item.setDetail(description)
-      item.setFilterText(
-        completion.filterText.getOrElse(completion.label)
-      )
-      item.setTextEdit(nameEdit)
-      item.setAdditionalTextEdits(
-        (completion.additionalEdits ++ additionalEdits).asJava
-      )
+      item.setFilterText(completion.filterText.getOrElse(completion.label))
+      item.setTextEdit(textEdit)
+      item.setAdditionalTextEdits((completion.additionalEdits ++ additionalEdits).asJava)
       completion.insertMode.foreach(item.setInsertTextMode)
 
       completion

presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionSuite.scala

@@ -1500,3 +1500,47 @@ class CompletionSuite extends BaseCompletionSuite:
         |""".stripMargin,
    )
 
+  @Test def `prepend-instead-of-replace` =
+    checkEdit(
+      """|object O:
+         |  printl@@println()
+         |""".stripMargin,
+      """|object O:
+         |  printlnprintln()
+         |""".stripMargin,
+      assertSingleItem = false
+    )
+
+  @Test def `prepend-instead-of-replace-duplicate-word` =
+    checkEdit(
+      """|object O:
+         |  println@@println()
+         |""".stripMargin,
+      """|object O:
+         |  printlnprintln()
+         |""".stripMargin,
+      assertSingleItem = false
+    )
+
+  @Test def `replace-when-inside` =
+    checkEdit(
+      """|object O:
+         |  print@@ln()
+         |""".stripMargin,
+      """|object O:
+         |  println()
+         |""".stripMargin,
+      assertSingleItem = false
+    )
+
+  @Test def `replace-exact-same` =
+    checkEdit(
+      """|object O:
+         |  println@@()
+         |""".stripMargin,
+      """|object O:
+         |  println()
+         |""".stripMargin,
+      assertSingleItem = false
+    )
+