Ensure extra document updates are not scheduled during updateSync

When changing the editor styles, we force the component to remeasure
character dimensions. If they change, each line's height could change
too, causing the current scroll top position to not match the viewport
the user was observing. Thus, when detecting a line height change, we
try to show users the area of the screen they were looking prior to
tweaking the font size.

In trying to maintain the aforementioned logical position, however, we
were mistakenly scheduling a new update before actually finishing the
current one. This was problematic because if the first update detected
that the longest screen line changed and such line was off-screen, it
would try to render it. Before having the chance to measure it, though,
the new update would kick in and delete the new longest screen line
node, because it assumed it had already been measured. Finally, when
`measureContentDuringUpdateSync` fired, it would notice that the longest
screen line node did not exist and throw an exception as a result.

This commit changes the `updateSync` method to set the `updateScheduled`
flag only before returning control to the caller, as opposed to doing so
at the beginning. This prevents calls to `scheduleUpdate` made in
`updateSync` from scheduling new unwanted updates.

spec/text-editor-component-spec.js

@@ -3610,6 +3610,23 @@ describe('TextEditorComponent', () => {
       element.style.display = 'none'
       await component.getNextUpdatePromise()
     })
+
+    it('does not throw an exception when the editor is soft-wrapped and changing the font size changes also the longest screen line', async () => {
+      const {component, element, editor} = buildComponent({rowsPerTile: 3, autoHeight: false})
+      editor.setText(
+        'Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do\n' +
+        'eiusmod tempor incididunt ut labore et dolore magna' +
+        'aliqua. Ut enim ad minim veniam, quis nostrud exercitation'
+      )
+      editor.setSoftWrapped(true)
+      await setEditorHeightInLines(component, 2)
+      await setEditorWidthInCharacters(component, 56)
+      await setScrollTop(component, 3 * component.getLineHeight())
+
+      element.style.fontSize = '20px'
+      TextEditor.didUpdateStyles()
+      await component.getNextUpdatePromise()
+    })
   })
 
   describe('synchronous updates', () => {

src/text-editor-component.js

@@ -213,15 +213,17 @@ class TextEditorComponent {
   }
 
   updateSync (useScheduler = false) {
-    this.updateScheduled = false
-
     // Don't proceed if we know we are not visible
-    if (!this.visible) return
+    if (!this.visible) {
+      this.updateScheduled = false
+      return
+    }
 
     // Don't proceed if we have to pay for a measurement anyway and detect
     // that we are no longer visible.
     if ((this.remeasureCharacterDimensions || this.remeasureAllBlockDecorations) && !this.isVisible()) {
       if (this.resolveNextUpdatePromise) this.resolveNextUpdatePromise()
+      this.updateScheduled = false
       return
     }
 
@@ -230,6 +232,7 @@ class TextEditorComponent {
     if (useScheduler && onlyBlinkingCursors) {
       this.refs.cursorsAndInput.updateCursorBlinkSync(this.cursorsBlinkedOff)
       if (this.resolveNextUpdatePromise) this.resolveNextUpdatePromise()
+      this.updateScheduled = false
       return
     }
 
@@ -266,6 +269,8 @@ class TextEditorComponent {
       this.measureContentDuringUpdateSync()
       this.updateSyncAfterMeasuringContent()
     }
+
+    this.updateScheduled = false
   }
 
   measureBlockDecorations () {