Properly handle regions that straddle existing regions

When we're creating folds that contain other folds, we'll need the
region based on the new fold's row mapping to overwrite the regions
from the old folds. This commit ensures that the new region cleanly
slots in, replacing any regions it completely contains and splitting
regions that it only partially overlaps.

src/app/row-map.coffee

@@ -29,30 +29,39 @@ class RowMap
   mapBufferRowRange: (startBufferRow, endBufferRow, screenRows) ->
     { index, bufferRow, screenRow } = @traverseToBufferRow(startBufferRow)
 
+    overlapStartIndex = index
+    overlapStartBufferRow = bufferRow
+    overlapEndIndex = index
+    overlapEndBufferRow = bufferRow
+    overlapEndScreenRow = screenRow
+
+    # determine regions that the new region overlaps. they will need replacement.
+    while overlapEndIndex < @regions.length
+      region = @regions[overlapEndIndex]
+      overlapEndBufferRow += region.bufferRows
+      overlapEndScreenRow += region.screenRows
+      break if overlapEndBufferRow >= endBufferRow
+      overlapEndIndex++
+
+    # we will replace overlapStartIndex..overlapEndIndex with these regions
     newRegions = []
 
-    preRows = startBufferRow - bufferRow
+    # if we straddle the first overlapping region, push a smaller region representing
+    # the portion before the new region
+    preRows = startBufferRow - overlapStartBufferRow
     if preRows > 0
       newRegions.push(bufferRows: preRows, screenRows: preRows)
 
-    bufferRows = endBufferRow - startBufferRow
-    newRegions.push({bufferRows, screenRows})
+    # push the new region
+    newRegions.push(bufferRows: endBufferRow - startBufferRow, screenRows: screenRows)
 
-    startIndex = index
-    endIndex = index
-    while bufferRows > 0 and endIndex < @regions.length
-      region = @regions[endIndex]
-      if region.bufferRows < bufferRows
-        bufferRows -= region.bufferRows
-        endIndex++
-      else
-        postBufferRows = region.bufferRows - preRows - bufferRows
-        postScreenRows = region.screenRows - preRows - screenRows
-        if postBufferRows > 0 or postScreenRows > 0
-          newRegions.push(bufferRows: postBufferRows, screenRows: postScreenRows)
-        bufferRows = 0
+    # if we straddle the last overlapping region, push a smaller region representing
+    # the portion after the new region
+    if overlapEndBufferRow > endBufferRow
+      endScreenRow = screenRow + preRows + screenRows
+      newRegions.push(bufferRows: overlapEndBufferRow - endBufferRow, screenRows: overlapEndScreenRow - endScreenRow)
 
-    @regions[startIndex..endIndex] = newRegions
+    @regions[overlapStartIndex..overlapEndIndex] = newRegions
 
   applyBufferDelta: (startBufferRow, delta) ->
     { region } = @traverseToBufferRow(startBufferRow)