Borders don't work with sliding objects as you have found. The simplest compromise is to remove the borders and put a horizontal line in place of the bottom border, set to slide up and resitze the enclosing part just like your fields. This doesn't give you side borders--that doesn't really work well, but does give you side borders.
Vertical lines that extend into the next part down (Footer or grand summary layout part) by a single pixel can be used as side borders--at least that worked in v12--haven't tried it recently and the exact method that works changes with the version of filemaker. But even when it works, that extra pixel produces a less than perfect result in the look of your report.
Key facts about sliding layout objects:
- It's only visible in preview mode and when you print/save as PDF...
- Sliding fields will shrink but not expand.
- All layout objects below and in the same layout part as the slide/resize field need to also be set to slide up and resize.
- Objects in headers and footers will not slide.
- Portals will shrink/slide to fit the number of rows of records, but fields within the portal row will not shrink/slide.
- Fields will slide up only if Top alignment is specified for it and will slide left only if Left alignment is specified.
- Consistent side borders are difficult to achieve with sliding fields.
- In FMP13, hidden objects (”Hide object when”) will slide/resize.