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Level 2 of the CSS Grid Layout specification includes a
subgrid value for
grid-template-rows. The value is currently being implemented in Firefox and this guide details what subgrid does, and gives some use cases and design patterns that are solved by the feature.
This feature is not yet in any browser for testing. This guide has been written in preparation for the feature to ship in Firefox Nightly in order that web developers can test it.
Introduction to subgrid
When you add
display: grid to a grid container, only the direct children become grid items and can then be placed on the grid that you have created. The children of these items display in normal flow.
You can "nest" grids, by making a grid item a grid container. These grids however are independent of the parent grid and of each other, meaning that they do not take their track sizing from the parent grid. This makes it difficult to line items up with a main grid.
If you give the value
grid-template-rows or both, then instead of creating a new track listing, the nested grid uses the tracks defined on the parent. If you use
grid-template-columns: subgrid and the nested grid spans three column tracks of the parent, the nested grid will have three column tracks of the same size as the parent grid. Gaps are inherited but can also be overridden with a different gap value. Line names can be passed from the parent into the subgrid, the subgrid can also declare its own line names.
Subgrid for columns
In the example below I have a grid layout with nine
1fr column tracks and four rows which are a minimum of 100px tall.
.item from column line 2 to 7, and row 2 to 4. I then make this grid item into a grid, giving it column tracks which are a subgrid and defining rows as normal. As the item spans five column tracks, this means that the subgrid has five column tracks. I can then place
.subitem on this grid.
The rows in this example are not a subgrid and so behave as a nested grid does normally. The grid area on the parent expands to be large enough for this nested grid.
Note that line numbering restarts inside the subgrid. Therefore column line 1 when inside the subgrid, is the first line of the subgrid. The subgridded element doesn't inherit the line numbers of the parent grid. This means that you can safely lay out a component that may be placed in different positions on the main grid, knowing that the line numbers on the component will always be the same.
Subgrid for rows
The next example is the same setup, however we are using subgrid as the value of
grid-template-rows and defining explicit column tracks. So the column tracks behave as a regular nested grid, but the rows are tied to the two tracks that the child spans.
A subgrid in both dimensions
You can of course define boh rows and columns as a subgrid, as in the example below. This means that your subgrid is tied in both dimensions to the number of tracks on the parent.
No implicit grid in a subgridded dimension
If you need to autoplace items, and do not know how many items you will have, take care when creating a subgrid which would prevent additional rows being created to hold those items. Take a look at the next example, it uses the same parent and child grid as in the example above, however inside the subgrid I have twelve items trying to autoplace into ten grid cells. As the subgrid is on both dimensions there is nowhere for the extra two items to go and so they go into the last track of the grid, as defined in the specification.
If we remove the grid-template-rows value we enable regular creation of implicit tracks, and although these won't line up with the tracks of the parent, as many as are required will be created.
The gap properties and subgrid
If you have a
row-gap specified on the parent, this will be passed through to the subgrid, so it will have the same spacing between tracks as the parent. In some situations you may wish the subgrid tracks to have a different gap or no gap. This can be achieved by using the gap properties on the grid container of the subgrid.
You can see this in the example below. The parent grid has a gap of 20px for rows and columns. The subgrid has
row-gap set to
If you inspect this in the Firefox Grid Inspector you can see how the line of the grid is in the correct place down the center of the gap, so when we set the gap to 0, it acts in a similar way to applying a negative margin to an element. Giving the space from the gap back to the item.
Named grid lines
When using CSS Grid you can name lines on your grid and then position items based on those names rather than the line number. The line names on the parent grid are passed into the subgrid, and you can place items using them. In the below example I have named lines on the parent
col-end and then used those to place the subitem.
You can also specify line names on the subgrid. This is achieved by adding a list of line names enclosed in square brackets after the
subgrid keyword. If you have four lines in your subgrid, to name them all you could use the syntax
grid-template-columns: subgrid [line1] [line2] [line3] [line4]
Line specified on the subgrid are added to any lines specified on the parent so you can use either or both. To demonstrate this, I have positioned one item in the example below using the parent lines, and one using the subgrid lines.
Other than needing to take care of items which do not fit in your subgrid, then a subgrid acts very similarly to any nested grid, the only difference is that the track sizing of the subgrid is set on the parent grid. As with any nested grid however, the size of content in the subgrid can change the track sizing, assuming a track sizing method is used which allows content to affect the size, therefore auto-sized row tracks for example will grow to fit content in the main grid and content in the subgrid.
As the subgrid value acts in much the same way as a regular nested grid, it is easy to switch between the two. For example if you realise that you need an implicit grid for rows, all you would need to do is remove the
subgrid value of
grid-template-rows and perhaps give a value for
grid-auto-rows to control the implicit track sizing.