Merge pull request #5951 from atom/as-visible-row-range

Adjust `DisplayBuffer#getVisibleRowRange` logic

spec/display-buffer-spec.coffee

@@ -1247,3 +1247,28 @@ describe "DisplayBuffer", ->
 
       expect(displayBuffer.getScrollWidth()).toBe 10 * 63 + operatorWidth * 2 + cursorWidth
       expect(changedSpy.callCount).toBe 1
+
+  describe "::getVisibleRowRange()", ->
+    beforeEach ->
+      displayBuffer.setLineHeightInPixels(10)
+      displayBuffer.setHeight(100)
+
+    it "returns the first and the last visible rows", ->
+      displayBuffer.setScrollTop(0)
+
+      expect(displayBuffer.getVisibleRowRange()).toEqual [0, 9]
+
+    it "includes partially visible rows in the range", ->
+      displayBuffer.setScrollTop(5)
+
+      expect(displayBuffer.getVisibleRowRange()).toEqual [0, 10]
+
+    it "returns an empty range when lineHeight is 0", ->
+      displayBuffer.setLineHeightInPixels(0)
+
+      expect(displayBuffer.getVisibleRowRange()).toEqual [0, 0]
+
+    it "ends at last buffer row even if there's more space available", ->
+      displayBuffer.setScrollTop(60)
+
+      expect(displayBuffer.getVisibleRowRange()).toEqual [6, 13]

src/display-buffer.coffee

@@ -348,12 +348,13 @@ class DisplayBuffer extends Model
   getScrollWidth: ->
     @scrollWidth
 
+  # Returns an {Array} of two numbers representing the first and the last visible rows.
   getVisibleRowRange: ->
     return [0, 0] unless @getLineHeightInPixels() > 0
 
-    heightInLines = Math.ceil(@getHeight() / @getLineHeightInPixels()) + 1
     startRow = Math.floor(@getScrollTop() / @getLineHeightInPixels())
-    endRow = Math.min(@getLineCount(), startRow + heightInLines)
+    endRow = Math.ceil((@getScrollTop() + @getHeight()) / @getLineHeightInPixels()) - 1
+    endRow = Math.min(@getLineCount(), endRow)
 
     [startRow, endRow]