The fill tool was excluding fill-only materials when computing the image size for the boundary render. Removing this stroke filter extends the flood fill area to include the strokes with fill materials.
Note that all curves are rendered as strokes without fill triangulation for the purposes of the fill tool. This is the same behavior as in GPv2. The consequence is that for most fill strokes, which are _not actually cyclic and closed_ there may be non-obvious open boundaries which lead to the flood fill covering the interior of the fill strokes (the inside edges of the flower petals are open). This is usually ok because the flood fill is rendered behind existing strokes. If necessary we could simply render all the boundaries as cyclic for fill materials, but that goes beyond a simple regression fix.
Pull Request: https://projects.blender.org/blender/blender/pulls/132746
f5eb916a08