Actually this position CRA is taking is not recent one, even before Covid, I heard cases where CRA was not giving full credit of the taxes remitted to IRS, if they determine you are more of a remote worker. The question here is not who taxes you first rather who gets to collect all or the majority of taxes.Makes sense. Thank you for your input!
The CPA said that recently CRA's stance is this: since the economic activity is being performed from within Canada, even if it is for a US employer (For H1B remote from Canada), then the income is considered "Canadian Sourced", even if the employer has no presence in Canada.
So when we file with the US using 1040-NR we can exclude the income earned from working in Canadian soil (essentially get the tax refund because it is payroll deducted) and get the refund from IRS (which will be filed first)
Then we file with CRA claiming any FTC for taxes paid while working on US soil that is taxable in Canada due to tax resident status.
Now, if we work for X days from the US in a given Tax Year, we can claim those taxes paid for X days as FTC with because it is then not Canadian Sourced (but Canadian residents have to pay on Global Income: This is where FTC prevents double taxation)
You may ask, how is a US salary not considered "US Sourced". See this IRS link. For salaries and wages, our USD salary is considered "US Sourced" based on where the services are performed. So if you work from Canada remotely, then the income is not US sourced.
For commuters, what you are saying is 100% right. In that case, the "where" is on US soil as they commute to US and come back.
If you are daily traveller, you are are fine, you still file with IRS and then claim that as foreign tax credit in your CRA filling. But if you are more of a remote worker, this will be increasingly challenging. I say increasingly because, I personally know few cases who have done this in past without any issues.
Have you explored W8BEN
What Is a W-8BEN Form? (velocityglobal.com)
Basically that reduces withholding or create exemptions, then you remit that to CRA instead of IRS. Now if you already paid taxes as remote worker to IRS, you have to claim that back then remit to CRA. little more complicated.
US employer may be reluctant as well, because then they enter into employer-employee relationship with someone who is residing in a foreign country, besides taxes there are other consequences.
Best is to convert to contractor and not remain full time employee, saves every one headache, if you are planning to be fully remote.