It seems possible that as soon as JFK got in the limousine he jammed himself right in the corner of the seating to give his bad back support for the journey ahead. This snap from Love Field, taken soon after he gets in the limo shows the back of his smooth-fitting jacket all creased and bunched up.
We know from the Croft photo it stays like this for nearly the the whole journey until seconds before the shooting. To imagine it then straightens itself out seconds after the Croft photo and just before the shooting starts seems silly, to say the least. The Willis and Betzner blow-ups show no fine detail, they're just blocks of colour and can be ignored.
We see what we want to see I suppose. Some see 'modest' or 'moderate' bunching, as if to downplay its significance, I see 'significant' bunching, easily enough to move the hole in the jacket down a few inches when in the correct position, however, I still think the SBT is baloney but for other reasons.
Bringing the shirt into it is baffling. We cannot see it, we have no idea what is happening with it. We have photographic evidence it can bunch up just as much as the jacket.
To find out how much the jacket bunched up compare it to the autopsy photos of the back wound. To find out how much the shirt bunched up compare it to the jacket.