Highlighter area coordinates offset by scrolling

Environment

• Ubuntu 16.04
• Chromium 66
• adblockplusui c571bfe8

Steps to reproduce

1. Run npm run $ test.io-highlighter to create smoke tests
2. Open smoke/io-highlighter.html
3. Run document.body.style.height = "200vh" to make page scrollable
4. Scroll down a bit
5. Click on either "Highlight" or "Hide"
6. Draw an area on the canvas

Observed behavior

Area is drawn at an offset to the top of the cursor.

Expected behavior

Area is drawn where the cursor is.