This may be something that others are already doing, but since it's often an issue, I thought I'd share a solution to this age old issue with columnar reports and sliding in preview mode.
The issue is when the column data is highly variable and sometimes shrinks to 0-1 line of text and sometimes 10 lines. Using traditional lines or borders on fields often results in gaps at the bottom of the row when smaller objects slide up with sliding turned on.
TL;DR -- Extend the part hight beyond what you need, make the vertical lines way longer than the data fields (ex. 200 pt.), then using the Inspector, manually force the part height back to the desired max size (Ex. 55 pts.).
This technique uses the new behaviour of Layout parts truncating their contents so that the line extends as long as you need it. Interestingly, this same behavior broke the old workaround for doing this where you would put really long lines in the header to trail down over the body rows!
The main downside is in layout mode, the lines still display and are clickable within the parts that follow below it.
STEP 1 - Make the lines longer than everything else
Step 2 - Manually set the part height back to the desired size.
Result - Weep with joy!
Hope this helps others!