Right arrow doesn't consistently complete a tag suggestion

[qualitymanifest]

Found this while working on #1853
If you are typing a tag for which there is a suggestion, pressing right arrow doesn't usually work to complete it.

Expected

Pressing right arrow should complete a tag suggestion

Observed

The only way it works is if you "back up" the cursor a character or two, then hit right arrow repeatedly as fast as you can.

Reproduced

1. Type out a few characters of a tag that you already use somewhere so that a suggestion pops up
2. Tap the right arrow button. The tag does not complete.
3. Left arrow back a few characters
4. Full speed ahead with the right arrow. Usually you can get it to complete

I believe the issue is in tag-input's interceptRightArrow handler. caretPosition should never equal value.length since value.length is 1-offset and caretPosition is 0-offset. I'm guessing the reason why it works when you press right arrow really fast is that something funky is happening with the cursor position state and it hasn't had time to correct itself.

Make	Model	OS Version	App Version
Asus	Zenbook	Linux Mint 19	Current dev branch... App says 1.12.0 but I think it's 1.14.0?