Adjust DisplayBuffer#getVisibleRowRange logic

This fixes #4596, where the calculation returned a wrong range.

* 📝 State clearly that we'll return a closed interval in docs
* ✅ Write tests to ensure a correct behavior

spec/display-buffer-spec.coffee

@@ -1247,3 +1247,22 @@ 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 a closed interval of visible rows", ->
+      displayBuffer.setScrollTop(0)
+
+      expect(displayBuffer.getVisibleRowRange()).toEqual [0, 9]
+
+    it "includes partially visible rows in the interval", ->
+      displayBuffer.setScrollTop(5)
+      expect(displayBuffer.getVisibleRowRange()).toEqual [0, 10]
+
+    it "returns an empty interval when lineHeight is 0", ->
+      displayBuffer.setLineHeightInPixels(0)
+
+      expect(displayBuffer.getVisibleRowRange()).toEqual [0, 0]

src/display-buffer.coffee

@@ -348,12 +348,13 @@ class DisplayBuffer extends Model
   getScrollWidth: ->
     @scrollWidth
 
+  # Returns an {Array} of two numbers representing a closed interval of 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]