Handle changing ::scrollTop in TextEditorPresenter

spec/text-editor-presenter-spec.coffee

@@ -78,3 +78,39 @@ describe "TextEditorPresenter", ->
         }
 
         # rows beyond the end of the content are not rendered
+
+    describe "when ::scrollTop changes", ->
+      it "updates the lines that are visible on screen", ->
+        presenter = new TextEditorPresenter(model: editor, clientHeight: 25, scrollTop: 0, lineHeight: 10, lineOverdrawMargin: 1)
+        presenter.setScrollTop(25)
+
+        line0 = editor.tokenizedLineForScreenRow(0)
+        expect(presenter.state.lines[line0.id]).toBeUndefined()
+
+        line1 = editor.tokenizedLineForScreenRow(1)
+        expectValues presenter.state.lines[line1.id], {
+          screenRow: 1
+          tokens: line1.tokens
+          top: 10 * 1
+        }
+
+        line2 = editor.tokenizedLineForScreenRow(2)
+        expectValues presenter.state.lines[line2.id], {
+          screenRow: 2
+          tokens: line2.tokens
+          top: 10 * 2
+        }
+
+        line3 = editor.tokenizedLineForScreenRow(3)
+        expectValues presenter.state.lines[line3.id], {
+          screenRow: 3
+          tokens: line3.tokens
+          top: 10 * 3
+        }
+
+        line4 = editor.tokenizedLineForScreenRow(4)
+        expectValues presenter.state.lines[line4.id], {
+          screenRow: 4
+          tokens: line4.tokens
+          top: 10 * 4
+        }

src/text-editor-presenter.coffee

@@ -6,15 +6,45 @@ class TextEditorPresenter
 
   buildLinesState: ->
     @state.lines = {}
-    startRow = Math.floor(@scrollTop / @lineHeight) - @lineOverdrawMargin
-    startRow = Math.max(0, startRow)
-    endRow = startRow + Math.ceil(@clientHeight / @lineHeight) + @lineOverdrawMargin
-    endRow = Math.min(@model.getScreenLineCount(), endRow)
+    @updateLinesState()
 
-    for line, i in @model.tokenizedLinesForScreenRows(startRow, endRow)
-      row = startRow + i
-      @state.lines[line.id] = {
-        screenRow: row
-        tokens: line.tokens
-        top: row * @lineHeight
-      }
+  updateLinesState: ->
+    visibleLineIds = {}
+    endRow = @getEndRow()
+    startRow = @getStartRow()
+
+    row = startRow
+    while row < endRow
+      line = @model.tokenizedLineForScreenRow(row)
+      visibleLineIds[line.id] = true
+      if @state.lines.hasOwnProperty(line.id)
+        @updateLineState(row, line)
+      else
+        @buildLineState(row, line)
+      row++
+
+    for id, line of @state.lines
+      unless visibleLineIds.hasOwnProperty(id)
+        delete @state.lines[id]
+
+  updateLineState: (row, line) ->
+    lineState = @state.lines[line.id]
+    lineState.screenRow = row
+    lineState.top = row * @lineHeight
+
+  buildLineState: (row, line) ->
+    @state.lines[line.id] =
+      screenRow: row
+      tokens: line.tokens
+      top: row * @lineHeight
+
+  getStartRow: ->
+    startRow = Math.floor(@scrollTop / @lineHeight) - @lineOverdrawMargin
+    Math.max(0, startRow)
+
+  getEndRow: ->
+    endRow = @getStartRow() + Math.ceil(@clientHeight / @lineHeight) + @lineOverdrawMargin
+    Math.min(@model.getScreenLineCount(), endRow)
+
+  setScrollTop: (@scrollTop) ->
+    @updateLinesState()