Miscalculation when using keepWithNext

Hi,

We've run in to an issue with ReportLab where it can orphan a heading 
under some circumstances, despite things being correctly marked with 
keepWithNext.

After some digging we found that flowables._listWrapOn() does not 
compute the same height as Frame._add(). So what happens is that 
KeepTogether things the flowables will all fit, but once the Frame 
starts adding things the last element gets punted to the next page.

I'm afraid I don't have an easy example for you, but I do have some 
numbers that should allow anyone to go through the code.

We have the following elements that are all marked keepWithNext:

Width   Height   Space Before    Space After
--------------------------------------------
0       3        0               0
470     15       12              6
470     36       6               0

The frame is at y=67. The space required for the above elements is 
3+12+15+6+36=72. I.e. it won't fit. However _listWrapOn() computes a 
required height of 57 here, so it thinks things will fit.

There are two bugs in play here:

a) _listWrapOn() ignores things with 0 width, however Frame._add() does 
not. So the first element gets discarded, dropping 3 from the resulting 
height.

b) _listWrapOn() assumes it is at the top of a frame, and should 
therefore discard the first flowable's spaceBefore. This seems unsafe as 
it will then often underestimate the space required. It would likely be 
better to assume it is never at the top and hence overestimate. This bug 
then acts on the second flowable (since the first was dropped), dropping 
another 12 from the total height.

And 72 - 12 - 3 = 57, and we get the bogus height number.

For now I've worked around this by making sure that initial flowable is 
1×3 instead of 0×3.

I also noticed that KeepTogether._H0 is probably incorrect since it 
doesn't include spaceBefore. But that might be tied to the assumption of 
always being at the top of a frame.

Regards
-- 
Pierre Ossman           Software Development
Cendio AB               https://cendio.com
Teknikringen 8          https://twitter.com/ThinLinc
583 30 Linköping        https://facebook.com/ThinLinc
Phone: +46-13-214600

A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
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