Inserting a mapping within an existing mapping preserves its shape

Inserting a mapping should never change the position of any existing
mappings on screen or in the buffer. It's simply a statement about a
range of rows in the buffer mapping to a range of existing rows on
screen, but shouldn't add or remove any rows. Adding and removing rows
on screen or in the buffer is the job of the applyBufferDelta and
applyScreenDelta methods.

src/app/row-map.coffee

@@ -11,6 +11,13 @@ class RowMap
       screenRow += targetBufferRow - bufferRow
       [screenRow, screenRow + 1]
 
+  bufferRowRangeForBufferRow: (targetBufferRow) ->
+    { mapping, screenRow, bufferRow } = @traverseToBufferRow(targetBufferRow)
+    if mapping and mapping.bufferRows != mapping.screenRows # 1:n mapping
+      [bufferRow, bufferRow + mapping.bufferRows]
+    else                                                    # 1:1 mapping
+      [targetBufferRow, targetBufferRow + 1]
+
   bufferRowRangeForScreenRow: (targetScreenRow) ->
     { mapping, screenRow, bufferRow } = @traverseToScreenRow(targetScreenRow)
     if mapping and mapping.bufferRows != mapping.screenRows # 1:n mapping
@@ -20,24 +27,33 @@ class RowMap
       [bufferRow, bufferRow + 1]
 
   mapBufferRowRange: (startBufferRow, endBufferRow, screenRows) ->
-    { mapping, index, bufferRow } = @traverseToBufferRow(startBufferRow)
-
-    if mapping and mapping.bufferRows != mapping.screenRows
-      if bufferRow == startBufferRow and bufferRow + mapping.bufferRows == endBufferRow
-        mapping.screenRows = screenRows
-        return
-      else
-        throw new Error("Invalid mapping insertion")
-
-    padBefore = startBufferRow - bufferRow
-    padAfter = (bufferRow + mapping?.bufferRows) - endBufferRow
+    { mapping, index, bufferRow, screenRow } = @traverseToBufferRow(startBufferRow)
 
     newMappings = []
-    newMappings.push(bufferRows: padBefore, screenRows: padBefore) if padBefore > 0
-    newMappings.push(bufferRows: endBufferRow - startBufferRow, screenRows: screenRows)
-    newMappings.push(bufferRows: padAfter, screenRows: padAfter) if padAfter > 0
+
+    preRows = startBufferRow - bufferRow
+    if preRows > 0
+      newMappings.push(bufferRows: preRows, screenRows: preRows)
+
+    bufferRows = endBufferRow - startBufferRow
+    newMappings.push({bufferRows, screenRows})
+
+    if mapping
+      postBufferRows = mapping.bufferRows - preRows - bufferRows
+      postScreenRows = mapping.screenRows - preRows - screenRows
+      if postBufferRows > 0 or postScreenRows > 0
+        newMappings.push(bufferRows: postBufferRows, screenRows: postScreenRows)
+
     @mappings[index..index] = newMappings
 
+  applyBufferDelta: (startBufferRow, delta) ->
+    { mapping } = @traverseToBufferRow(startBufferRow)
+    mapping?.bufferRows += delta
+
+  applyScreenDelta: (startBufferRow, delta) ->
+    { mapping } = @traverseToScreenRow(startBufferRow)
+    mapping?.screenRows += delta
+
   traverseToBufferRow: (targetBufferRow) ->
     bufferRow = 0
     screenRow = 0