Yikes, sorry to hear that (I am not sure how mine will be dealt with - US citizen in Canada for the first full year next year) - I've been told I will be taxed whichever is higher...
Do you know how Roth IRA is treated in Canada? (I hope they recognize it that it is after-tax contribution but I am getting the feeling they will tax the withdrawal somehow...)
Your Roth withdrawals will be tax free in Canada. But note the bit I underlined about contributions after you moved to Canada.
https://www.irs.gov/pub/irs-pdf/p597.pdf
Roth IRAs.
A distribution from a Roth IRA
is exempt from Canadian tax to the extent it
would be exempt from U.S. tax if paid to a U.S.
resident. In addition, you may elect to defer any
tax in Canada on income accrued within the
Roth IRA but not distributed by the Roth IRA.
However, you cannot defer tax on any accruals
due to contributions made after you become a
Canadian resident