a comma question - I know, it' depends.

I know this will depend on the style guide, but mine is strangely silent on this topic (or more likely, I'm crap at finding the information in the book).  So I'm wondering what you guys think.

If I sign a communication, saying thank you and then my name, I would often put:

blablabla
Thank you, R.

But what if I want to start the email by thanking someone else?  For example:

Thank you Bob, that is just what I needed to know.

To my mind, there would not be a comma between "thank you" and "Bob" because the thank you is acting both as an expression of gratitude and a stand-in for "dear".  I don't start a letter like this:

Dear, Bob,

That would be weird.

And yet, every spelling and grammar checker HATES me for saying "thank you bob,"  Multiple error messages and often autocorrections to put the comma between thank you and the name.

Is it just the Canadian in me that feels starting a sentence with "thank you person, and more words on the subject" is a normal way to start a sentence?

Just mildly bemused this morning, procrastinating, and thought this might be meaningless enough to post in the drivel form as comma placement always has lively discussion.

(for those who are curious, I use a variation on the Oxford Canadian style guide - with a emphasis on the UK side)

Is that the bloody serial comma?  

I would not use the additional comma in your example.  

Somewhere, you'll probably be able to find a rule that states indeed, a comma is proper there.

With that said, similar to a proper place setting at the dinner table, the salad fork goes RIGHT THERE, the teacup & saucer goes RIGHT THERE, etc., it's a guideline.  A recommendation.  

Thank you Bob, at the beginning, is correct.
Grammar check thinks you are using it at the end to sign it, so that use would be wrong.
One of the limitations of grammar check, it ignores too much context.  

Mrs. Washenfelder would say, always a comma after thank you.

I would also start "Thank you Bob, ..." I agree the spellcheck thinks you are ending "Thank you, Bob" where Bob is your name. It may not be right but I was taught to read the sentence, if you pause then there should be a comma if you don't pause there shouldn't be one.

I'm inclined to think that, although I would do as you R and just put

Thank you Bob,

at the start of a message that this is technically incorrect.

I found a website grammarhow that sort of explains the rule, and it is to do with whether you can remove part of the sentence. 'Thank you' is shortened from 'I thank you' which is a complete sentence.
So we could say

I thank Paul for creating Permies Forum

thank you for creating Permies forum

or

Thank you, Paul, for creating Permies forum

So

Thank you, Paul,

clear as mud right?!

What I learned is that when addressing someone directly, their name/title is always separated from the rest of the sentence with a comma or commas. The classic example of why it's important to do this is

Let's eat Grandma.
Vs
Let's eat, Grandma.

The salutation in a letter is an exception. The comma separating the person being addressed is excluded, but the salutation is on its own line. So your example could read

Thank you Bob,
That is just what I needed to know.
Or
Thank you, Bob, that is just what I needed to know.

The second version would be incorrect because it's a run-on sentence, though. You'd need to connect the two somehow, like, "Thank you, Bob, because that's just what I needed." That sounds pretty awkward, though. Maybe change the structure entirely to, "Thank you, Bob, for giving me the information I needed."  I'd just go with the salutation on its own line.

Just posting to say, as a descriptive linguist, I see commas, everywhere, whenever, anyone, feels, like, u,s,i,n,g, them ,,, and I think it's hilarious and I enjoy it,