Adjust multiple mappings if needed when applying screen deltas

If there are multiple mappings following the start row of the delta
that span fewer screen rows than the delta, we collapse mappings to 0
screen rows until we have removed the number of rows specified by the
delta.

Expanding positive deltas expands the first mapping following the
start row as it did previously.

src/app/row-map.coffee

@@ -50,9 +50,18 @@ class RowMap
     { mapping } = @traverseToBufferRow(startBufferRow)
     mapping?.bufferRows += delta
 
-  applyScreenDelta: (startBufferRow, delta) ->
-    { mapping } = @traverseToScreenRow(startBufferRow)
-    mapping?.screenRows += delta
+  applyScreenDelta: (startScreenRow, delta) ->
+    { index } = @traverseToScreenRow(startScreenRow)
+    until delta == 0
+      { bufferRows, screenRows } = @mappings[index]
+      screenRows += delta
+      if screenRows < 0
+        delta = screenRows
+        screenRows = 0
+      else
+        delta = 0
+      @mappings[index] = { bufferRows, screenRows }
+      index++
 
   traverseToBufferRow: (targetBufferRow) ->
     bufferRow = 0