don't use document for checking whether a folding region is one-line, use only PSI, as it may not correspond to the state of the document (EA-81838 - IOOBE: LineSet.findLineIndex)

idea/src/org/jetbrains/kotlin/idea/KotlinFoldingBuilder.kt

@@ -23,6 +23,7 @@ import com.intellij.lang.folding.FoldingDescriptor
 import com.intellij.openapi.editor.Document
 import com.intellij.openapi.project.DumbAware
 import com.intellij.openapi.util.TextRange
+import com.intellij.openapi.util.text.StringUtil
 import com.intellij.psi.PsiElement
 import com.intellij.psi.PsiWhiteSpace
 import org.jetbrains.kotlin.KtNodeTypes
@@ -61,7 +62,9 @@ class KotlinFoldingBuilder : CustomFoldingBuilder(), DumbAware {
     private fun appendDescriptors(node: ASTNode, document: Document, descriptors: MutableList<FoldingDescriptor>) {
         if (needFolding(node)) {
             val textRange = getRangeToFold(node)
-            if (!isOneLine(textRange, document)) {
+            val relativeRange = textRange.shiftRight(-node.textRange.startOffset)
+            val foldRegionText = node.chars.subSequence(relativeRange.startOffset, relativeRange.endOffset)
+            if (StringUtil.countNewLines(foldRegionText) > 0) {
                 descriptors.add(FoldingDescriptor(node, textRange))
             }
         }
@@ -93,9 +96,6 @@ class KotlinFoldingBuilder : CustomFoldingBuilder(), DumbAware {
         return node.textRange
     }
 
-    private fun isOneLine(textRange: TextRange, document: Document) =
-        document.getLineNumber(textRange.startOffset) == document.getLineNumber(textRange.endOffset)
-
     override fun getLanguagePlaceholderText(node: ASTNode, range: TextRange): String = when {
         node.elementType == KtTokens.BLOCK_COMMENT -> "/.../"
         node.elementType == KDocTokens.KDOC -> "/**...*/"