Fix highlight end pixel position calculation

Previously, we were calculating the position preceding block decorations
for the row following the end of the highlighted range, but that's
actually wrong. We just want the position following block decorations of
the end of the highlighted range, plus one line height. This prevents us
from incorrectly rendering the end of highlight after block decorations
that immediately follow the end of the highlighted range.

spec/text-editor-component-spec.js

@@ -1697,6 +1697,22 @@ describe('TextEditorComponent', () => {
       await component.getNextUpdatePromise()
       expect(Array.from(marker1Region.parentElement.children).indexOf(marker1Region)).toBe(0)
     })
+
+    it('correctly positions highlights that end on rows preceding block decorations', async () => {
+      const {editor, element, component} = buildComponent()
+
+      const item = document.createElement('div')
+      item.style.height = '30px'
+      editor.decorateMarker(editor.markBufferPosition([4, 0]), {
+        type: 'block',  position: 'after', item
+      })
+      editor.setSelectedBufferRange([[3, 0], [4, Infinity]])
+      await component.getNextUpdatePromise()
+
+      const regions = element.querySelectorAll('.selection .region')
+      expect(regions[0].offsetTop).toBe(3 * component.getLineHeight())
+      expect(regions[1].offsetTop).toBe(4 * component.getLineHeight())
+    })
   })
 
   describe('overlay decorations', () => {

src/text-editor-component.js

@@ -1316,7 +1316,7 @@ class TextEditorComponent {
         const {start, end} = highlight.screenRange
         highlight.startPixelTop = this.pixelPositionAfterBlocksForRow(start.row)
         highlight.startPixelLeft = this.pixelLeftForRowAndColumn(start.row, start.column)
-        highlight.endPixelTop = this.pixelPositionBeforeBlocksForRow(end.row + 1)
+        highlight.endPixelTop = this.pixelPositionBeforeBlocksForRow(end.row) + this.getLineHeight()
         highlight.endPixelLeft = this.pixelLeftForRowAndColumn(end.row, end.column)
       }
       this.decorationsToRender.highlights.set(tileRow, highlights)