A LayoutBuilder lets us figure out the height of the nav bar after stretching from overscrolls. This lets us determine how much to grow the size of the large title text during overscrolls.

This line is longer than 80 characters. Line breaking at strategic points may help readability too.

super nit: the logic is probably easier to see with clamp: (1.0 + (constraints.maxHeight - maxExtent) / maxExtent * 0.12)).clamp(1.0, 1.15),

Is the maxExtent here supposed to be minExtent? When constraints.maxHeight == minExtent the expression should evaluate to 1.0 from the looks of it?

Will this have hit testing problems when we scale the title? Transform.scale doesn't seem to change the layout size of the render object, so I assume when you change the scale, the maximum hit testing region is still the size of the render object oh nvm the container resizes.

it's just a graphical effect. It's not interactive and you can't keep it in the transient overscroll state.

Is it possible for an overlength title to get clipped or overlap the trailing widget?

Very good point. Worth checking what native iOS does when the large title is already horizontally long enough to almost overflow.

Hm. Good point.

Ok this is what I understand from the behavior:

The trailing widget in large title nav bars lies above the title, therefore it won't overlap horizontally. Because scaling only happens when the height of the nav bar increases, it can't grow large enough vertically to overlap the navigation items (trailing, leading.)

The title may scale up to 1.15, but it is bound to its horizontal margins. Meaning if the text is almost large enough to overflow or if it is truncated, the title text may ever so slightly scale up to the horizontal margins. If it is already exactly at its horizontal margins, it will not scale. I'll update the PR to handle this edge case.

@xster @LongCatIsLooong I'm having trouble setting the max-width condition for the scale transformation. My original thought was to have a condition that would check for the width of the large title and ensure it's less than the nav bar builder's constraints.maxWidth, but I can't find a way to get the width of the large title text itself. Any ideas?

ah doh, sorry I missed this somehow.

Yes, it's somewhat intentionally complicated because text layout is expensive, so the API is rather low level. There are 2 ways.

The canonical way is to use a RenderBox the way the FittedBox object uses a RenderBox. During the layout phase, it lets its child calculate its size then reacts accordingly in its own paint.

The slightly less complicated way is to "dry run" it using a TextPainter. There's a bit of that going on in the picker.