Editor masking issue produced by horizontal scrolling

[rat-moonshine]

As reported by @JoelProminic in #39 ,

I was able to reproduce the horizontal scroll issue like this:

1. Create a new FlexJS Project
2. Make sure that the project is using a FlexJS SDK
3. Add a long line of code with an error. The line needs to be long enough that it will require a horizontal scroll. I added an example below that will trigger both an error and a warning
4. Slide the horizontal scroll bar. You should see that the wavy lines do not move

	<fx:Script>
		<![CDATA[
		private var  foo:String = doesNotExist + "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua." + doesNotExist2 + " Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.";
		private var bar = "";
		]]>
	</fx:Script>

Unfortunately, I also noticed that the code overlaps with the sidebar.

Here is my test project, if you need it. I tested with Moonshine 1.6.0 on Mac OS 10.10.5:
2017_10_12__TestHorizontalScroll.zip

[rat-moonshine]

I found a solution, but I need to ensure some more test cases.

[JoelProminic]

I tested with:

• Maximized window
• Unmaximized window
• Changing the width of the editor window by dragging
• Testing with existing text that exceeded the scrollable threshold
• Adding new text to exceed the scrollable threshold

I did not see any problems. Additionally, I confirmed that the error and warning lines are still working properly through these tests (see #39).