Don't try to measure lines that don't exist

By the time that the animation frame is delivered, the requested
autoscroll
position could not exist anymore. This could cause the editor component
to measure a non-existent line and, as a result, throw an exception.

With this commit we will always ignore measurements for screen lines
that do not exist.

spec/text-editor-component-spec.js

@@ -24,7 +24,7 @@ document.registerElement('text-editor-component-test-element', {
   })
 })
 
-describe('TextEditorComponent', () => {
+fdescribe('TextEditorComponent', () => {
   beforeEach(() => {
     jasmine.useRealClock()
 
@@ -961,6 +961,14 @@ describe('TextEditorComponent', () => {
       expect(component.getScrollLeft()).toBe(component.getScrollWidth() - component.getScrollContainerClientWidth())
     })
 
+    it('does not try to measure lines that do not exist when the animation frame is delivered', async () => {
+      const {component, editor} = buildComponent({autoHeight: false, height: 30, rowsPerTile: 2})
+      editor.scrollToBufferPosition([11, 5])
+      editor.getBuffer().deleteRows(11, 12)
+      await component.getNextUpdatePromise()
+      expect(component.getScrollBottom()).toBe((10 + 1) * component.measurements.lineHeight)
+    })
+
     it('accounts for the presence of horizontal scrollbars that appear during the same frame as the autoscroll', async () => {
       const {component, element, editor} = buildComponent({autoHeight: false})
       const {scrollContainer} = component.refs

src/text-editor-component.js

@@ -332,7 +332,7 @@ class TextEditorComponent {
     this.derivedDimensionsCache = {}
     this.updateModelSoftWrapColumn()
     if (this.pendingAutoscroll) {
-      const {screenRange, options} = this.pendingAutoscroll
+      let {screenRange, options} = this.pendingAutoscroll
       this.autoscrollVertically(screenRange, options)
       this.requestHorizontalMeasurement(screenRange.start.row, screenRange.start.column)
       this.requestHorizontalMeasurement(screenRange.end.row, screenRange.end.column)
@@ -2097,7 +2097,12 @@ class TextEditorComponent {
     if (column === 0) return
 
     if (row < this.getRenderedStartRow() || row >= this.getRenderedEndRow()) {
-      this.requestExtraLineToMeasure(row, this.props.model.screenLineForScreenRow(row))
+      const screenLine = this.props.model.screenLineForScreenRow(row)
+      if (screenLine) {
+        this.requestExtraLineToMeasure(row, screenLine)
+      } else {
+        return
+      }
     }
 
     let columns = this.horizontalPositionsToMeasure.get(row)