Fix rendering bug when folds hide the vertical scrollbar w/ soft wrap on

spec/text-editor-component-spec.js

@@ -286,6 +286,31 @@ describe('TextEditorComponent', () => {
       expect(lineNumberNodeForScreenRow(component, 0).querySelector('.foldable')).toBeNull()
     })
 
+    it('gracefully handles folds that change the soft-wrap boundary by causing the vertical scrollbar to disappear (regression)', async () => {
+      const text =  ('x'.repeat(100) + '\n') + 'y\n'.repeat(28) + '  z\n'.repeat(50)
+      const {component, element, editor} = buildComponent({text, height: 1000, width: 500})
+
+      element.addEventListener('scroll', (event) => {
+        event.stopPropagation()
+      }, true)
+
+      editor.setSoftWrapped(true)
+      jasmine.attachToDOM(element)
+      await component.getNextUpdatePromise()
+
+      const firstScreenLineLengthWithVerticalScrollbar = element.querySelector('.line').textContent.length
+
+      setScrollTop(component, 620)
+      await component.getNextUpdatePromise()
+
+      editor.foldBufferRow(28)
+      await component.getNextUpdatePromise()
+
+      const firstLineElement = element.querySelector('.line')
+      expect(firstLineElement.dataset.screenRow).toBe('0')
+      expect(firstLineElement.textContent.length).toBeGreaterThan(firstScreenLineLengthWithVerticalScrollbar)
+    })
+
     it('shows the foldable icon on the last screen row of a buffer row that can be folded', async () => {
       const {component, element, editor} = buildComponent({text: 'abc\n  de\nfghijklm\n  no', softWrapped: true})
       await setEditorWidthInCharacters(component, 5)

src/text-editor-component.js

@@ -362,7 +362,7 @@ class TextEditorComponent {
       this.requestHorizontalMeasurement(screenRange.start.row, screenRange.start.column)
       this.requestHorizontalMeasurement(screenRange.end.row, screenRange.end.column)
     }
-    this.populateVisibleRowRange()
+    this.populateVisibleRowRange(this.getRenderedStartRow())
     this.populateVisibleTiles()
     this.queryScreenLinesToRender()
     this.queryLongestLine()
@@ -2096,14 +2096,29 @@ class TextEditorComponent {
     return marginInBaseCharacters * this.getBaseCharacterWidth()
   }
 
+  // This method is called at the beginning of a frame render to relay any
+  // potential changes in the editor's width into the model before proceeding.
   updateModelSoftWrapColumn () {
     const {model} = this.props
     const newEditorWidthInChars = this.getScrollContainerClientWidthInBaseCharacters()
     if (newEditorWidthInChars !== model.getEditorWidthInChars()) {
       this.suppressUpdates = true
+
+      const renderedStartRow = this.getRenderedStartRow()
       this.props.model.setEditorWidthInChars(newEditorWidthInChars)
-      // Wrapping may cause a vertical scrollbar to appear, which will change the width again.
+
+      // Relaying a change in to the editor's client width may cause the
+      // vertical scrollbar to appear or disappear, which causes the editor's
+      // client width to change *again*. Make sure the display layer is fully
+      // populated for the visible area before recalculating the editor's
+      // width in characters. Then update the display layer *again* just in
+      // case a change in scrollbar visibility causes lines to wrap
+      // differently. We capture the renderedStartRow before resetting the
+      // display layer because once it has been reset, we can't compute the
+      // rendered start row accurately. 😥
+      this.populateVisibleRowRange(renderedStartRow)
       this.props.model.setEditorWidthInChars(this.getScrollContainerClientWidthInBaseCharacters())
+
       this.suppressUpdates = false
     }
   }
@@ -2867,12 +2882,11 @@ class TextEditorComponent {
     }
   }
 
-  // Ensure the spatial index is populated with rows that are currently
-  // visible so we *at least* get the longest row in the visible range.
-  populateVisibleRowRange () {
+  // Ensure the spatial index is populated with rows that are currently visible
+  populateVisibleRowRange (renderedStartRow) {
     const editorHeightInTiles = this.getScrollContainerHeight() / this.getLineHeight()
     const visibleTileCount = Math.ceil(editorHeightInTiles) + 1
-    const lastRenderedRow = this.getRenderedStartRow() + (visibleTileCount * this.getRowsPerTile())
+    const lastRenderedRow = renderedStartRow + (visibleTileCount * this.getRowsPerTile())
     this.props.model.displayLayer.populateSpatialIndexIfNeeded(Infinity, lastRenderedRow)
   }