row span and auto positionning in Firefox

[ledahulevogyre]

Here I try to layout 8 divs automatically, based on their row span definition.
https://codepen.io/anon/pen/Nvxdjo
Row heights appears to adapt well for the first 5 rows. But then, all the subsequent rows seem to have the height of the 5th row. The consequences are the blank spaces in the 4th and 5th divs.

The behavior seems good on Blink. It compresses the rows/divs as I expect.

[rachelandrew]

I think the issue is in a difference in how Firefox is interpreting those empty rows vs. Chrome.

If I add grid-auto-rows: 50px we end up with the same display in both. I'm just trying to figure out who is correct.

Should the auto size rows collapse to zero? That's what Chrome is doing. Or, should the space be distributed? This is what Firefox does.

[ledahulevogyre]

Thanks Rachel.
Firefox does not seem to always distribute the space evenly either. Not all the rows (of divs 1, 2, and 3 for example) have the same height.
I guess this is where the answer is but I don't grasp every concepts.

[mrego]

I'd need to check the track sizing algorithm, but on a first sight it seems that Chromium is right as it doesn't increase the size of rows without a clear reason.

Check this reduced example: https://codepen.io/mrego/pen/wqWbOg
First 2 rows doesn't need to be bigger than 20px in total (10px each), but Firefox is increasing them somehow.

[rachelandrew]

Yes, my initial feeling was that Chrome was doing the right thing. If there was nothing at all in the rows they would be zero height. The fact something spans them probably shouldn't make any difference.

[ledahulevogyre]

Chrome has bugs too, imo.
Here is @mrego 's example a bit modified (third element added in first colum) : https://codepen.io/anon/pen/JyRPzz
It behaves weird to me, when i3 has a row span of 2 or 3 (higher values seem ok)

[svillar]

In the test case by @mrego Blink is clearly the one behaving correctly. Without checking the algorithm is obvious that we are not increasing the sizes of the tracks if not needed, something that is a basis for all the heuristics used in the algorithm.

Now the technical justification based on the algorithm. The relevant section is section 11.5, list item 2. where it says "Increase sizes to accommodate spanning items"

• After processing the 1st item (the one spanning 2 rows, the sizes of the rows is as follows:
  [10, 10] [10, 10] [0, inf]
• Now we process the 2nd item (the algorithm processes 1st the non-spanning items, then the ones spanning 2, then spanning 3...). There are 6 different steps, we'll go over them:
  1. We need to accomodate a 80px item, rows 1 and 2 sum 20px, so we have to distribute 60px. As tracks 1 and 2 cannot grow (they are both [10, 10]) we distribute everything to the 3rd track. Here I suspect FF does the wrong thing, it distributes 60px equally between the 3 rows.
  2. Does nothing
  3. Does nothing
  4. Does nothing
  5. Same as in 1, as tracks 1 and 2 have a fixed growth limit of 10, then we distribute 60px to the third row exclusively
  6. Does nothing

So the correct result is indeed [10,10] [10,10] [60, 60]

[svillar]

Regarding the other example posted by @ledahulevogyre although it looks a bit weird it's correct in Chrome and again a bug in Firefox. Technically it's more complicated to explain than the above case. The algorithm sizes tracks by grouping items by the amount of span, meaning that it processes first the non-spanning items, then the ones spanning 2, then spanning 3... But there is one important thing to take into account, and is that when computing the track sizes for a particular span level it must do it independently on the DOM order of each item, meaning that if I have 2 items spanning 3 tracks, I should get the same result independently on how they're specified in the DOM.

I've modified your example slightly. First I specified all the item positions so that autoplacement does not generate different results, and then created the very same example but switching the positions of items 2 and 3 in the source code. Since the track sizing algorithm should work independently of the order (https://drafts.csswg.org/css-grid/#distribute-extra-space), in Blink the result is the same in both cases as it should be, while in Firefox they behave differently.

See:
https://codepen.io/anon/pen/WEGpPe?editors=1100#0
https://codepen.io/anon/pen/VzKpgx?editors=1100#0

[rachelandrew]

Thanks for all the digging into this!

I have raised the two issues with Firefox:

1. https://bugzilla.mozilla.org/show_bug.cgi?id=1386921
2. https://bugzilla.mozilla.org/show_bug.cgi?id=1386932

I have linked to this discussion but please add comments to those reports if you feel I have not fully represented the issue at hand.

Also added all of this as issue #12 on this site.

[ledahulevogyre]

Thank you @svillar
What does "The algorithm sizes tracks by grouping items by the amount of span" mean concretely? How does this rule make the last track to have a non zero height ? I don't understand.

[svillar]

So I'll try to explain it using the test case you've posted. As mentioned after processing the item spanning 2 rows the row sizes look like this:

[10,10] [10,10] [0, inf] [0,inf] [0,inf]

Now you need to process the other two items which span 3 columns. Item 2 spans rows 1,2 & 3 and item 3 spans rows 3,4 & 5. As I mentioned the algorithm ensures that the order in which you process items should not affect the final result. So the way it works is that we get candidate sizes for the tracks for every single item and then we run a final pass setting the sizes that make everything valid. These are the candidate sizes computed for each item spanning 3 rows:

item 2 (height 80px) computes: [10,10] [10,10] [60, 60] [0,inf] [0,inf]
item 3 (height 20px) computes: [10,10] [10,10] [6, 6] [6, 6] [6, 6]

So now you combine both results and get [10,10] [10,10] [60, 60] [6, 6] [6, 6]. You could think, "oh but you don't need nonzero tracks 4&5 because the item 3 already fits in row 3". That's true but you've explicitly set that the item spans through 3 auto rows so the right thing to do is to distribute equally among these tracks.

Hope I have clarified your doubts

[ledahulevogyre]

I see now. Thanks for the explanation.
I think it's good that the algorithm isn't influenced by the DOM order.
But intuitively, it's the layout order that should be prevalent. That is to say, the items starting "higher" should be processed first.
@rachelandrew , do you think it's good to ask the spec to be corrected that way ?

[rachelandrew]

I don't think this is a spec issue, you are of course able to raise issues against the spec if you want to.

[svillar]

Not sure what you mean by "it's the layout order that should be prevalent" but what grid does is preciselly layout its tracks so that:
a) content sized tracks ensure that the largest min-content contribution from any item in the track is respected
b) track sizes should be as compact as possible, meaning that we should not grow them if not needed

[ledahulevogyre]

But b) is not respected in this case. Or I misunderstood (quite possibly).
@rachelandrew why do you think it's not an issue ? Do you think it's normal that a grid can be smaller if you only increase one span number ?