Debugging CSS Grid Layouts With Firefox Grid Inspector

Overlay Grid

The first section you will see in the layout panel is Overlay Grid, which will show you all the elements on the page with display: grid applied. You will be able to turn on the grid overlay on each grid by checking their respective checkbox. For now, only one grid overlay can be displayed at any one time, but having multiple grid overlays is a feature on the roadmap.

Toggle grid overlay (Large preview)

Every additional grid will have a different color from the default purple, but you are free to change the colors of your grid overlays by clicking on the colored circle on the right of each grid element. The grid overlay will show all the grid tracks and grid gaps of the selected grid.

Customize grid overlay color
Customize grid overlay color (Large preview)

Once you select a grid, a rendering of the selected grid will appear in the space below in the color of the grid overlay. This rendering will show you each section of the grid you defined and hovering over any section will highlight its corresponding area on the actual page. There will also be a tool-tip that shows you the line number of the row and column of the highlighted grid item.

Visual highlighting tool
Visual highlighting tool (Large preview)

Most of us would be in the Rules panel when examining the CSS on our site, and you can toggle the grid overlay from there as well. Select the element which has display: grid applied to it, and click on the waffle-like icon on the property. The options for display of line numbers or grid area names that have been set on the Layout Panel will be respected when the grid overlay is toggled in this manner.

Toggle Grid overlay via Rules panel
Toggle Grid overlay via Rules panel (Large preview)

Grid Display Settings

The next section on the panel is the Grid Display Settings, which allows us to toggle three options for now. The display of line numbers, the display of area names and the option to extend grid lines indefinitely.

The basic premise of how Grid works is that you first define a grid, then proceed to place your grid items within that grid. You can either place those grid items manually or let the browser do it for you. The position of the grid items depends on their grid-row and grid-column values. Grid lines are referred to by their numerical index which starts from 1.

Toggle line numbers
Toggle line numbers (Large preview)

Once Display line numbers is active, the selected grid overlay will display the line numbers of the grid in the color of each respective grid overlay. Each grid has their own numerical grid index which starts from 1, different grids will not share the same numerical grid index.

Line numbers cut off at the top edge
Line numbers cut off at the top edge (Large preview)
Line numbers cut off at the side edge
Line numbers cut off at the side edge (Large preview)

If your grid extends the width or height of the viewport, you will notice that the line numbers get cut off at the edges. The Mozilla team is aware of this issue and it is being tracked under Bug 1396666.

You can also define a grid using the grid-template-areas property, which gives us the ability to name the areas on our grids. The syntax for this property also provides a visualization of the grid structure in the CSS itself, making it easier to understand the layout of the grid from your code.

Take the following block of CSS for example:

.subgrid1 { display: grid; grid-template-columns: 1fr 1fr 1fr; grid-auto-rows: 5em; grid-template-areas: "apple banana pear" "grape watermelon pineapple" "strawberry peach kiwi"}

This creates a 3 by 3 grid and each section is named according to how they are laid out in the grid-template-areas property. If the sections are named, they will be displayed as a second tool-tip when you hover over the rendering of the grid in the layout panel.

Grid area name tool-tip
Grid area name tool-tip (Large preview)

We can also toggle the display of the grid area names on the grid overlay by checking that option in Grid Display Settings. According to Mozilla, this feature was inspired by CSS Grid Template Builder, which was created by Anthony Dugois.

Toggle grid area names
Toggle grid area names (Large preview)

Fun fact. You can use emojis for grid area names and everything will work just fine.

Emoji grid names
Emoji grid names (Large preview)

The last option you can toggle is to extend grid lines indefinitely. By default, the grid lines on each grid are constrained to within the bounds of the grid container. But sometimes it can be useful to see how the grid aligns in the context of the entire page.

Extend grid lines indefinitely
Extend grid lines indefinitely (Large preview)

A Better Box Model Tool

The Box Model section under the Layout Panel shows a rendering of the dimensions of the selected element, its padding, borders, and margins. In addition to that, it also shows the CSS properties that affect the position, size and geometry of the selected element, namely box-sizing, display, float, line-height, position, and z-index. A convenience when it comes to troubleshooting layout-related CSS issues.

The computed value of the height and width of the selected element and its current positioning value is displayed below the box model outline. You can also directly manipulate the margins, borders, and paddings of the element from the diagram itself.

Tweak layout properties
Tweak layout properties (Large preview)

If you use CSS lengths other than pixels, the DevTools will convert the values into pixels automatically based on the computed value, which is really nifty.

Grid Inspector Plays Well With Transforms

There are many instances where Grid will be used in combination with other CSS layout properties, like transforms, for example. The Grid Inspector tool plays well with CSS transforms and the grid overlay will adjust to it, however, the selected element has been transformed. You will be able to see how the Grid has been rotated, skewed, scaled or translated.

Grid Inspector and Transforms
Grid Inspector and Transforms (Large preview)

Help Make The Grid Inspector Tool Better

As with any other browser feature, the Grid Inspector tool will inevitably have bugs as the team is hard at work adding new features and polishing existing ones. If you do encounter any such issues, please raise an issue at Bugzilla, Mozilla’s bug tracking tool.

The metabug, which tracks all Grid Inspector related issues, is Bug 1181227. You can search for the term Grid Inspector to see the list of related bugs as well.

Grid Inspector bugs
Grid Inspector bugs (Large preview)

If you have any suggestions or feedback on the Grid Inspector tool or the Firefox DevTools in general, Mozilla is on Discourse. Or you can also tweet at @FirefoxDevTools.


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