Steps to reproduce:

I have made a sample https://github.com/RedAndBlu/report-bug-firefox
open the file bug.html

Actual results:

the grid doesn't display correctly but collapse.
but if you toggle on and off grid-area: side and grid-area: nav in the console it fix itself.
I tested it on android and linux

Expected results:

you can compare with chromium (it easier to see it visually)

The Bugbug bot thinks this bug should belong to the 'Core::Layout' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

I think this is essentially a dupe of bug 1310551.

Unfortunately, Firefox's implementation of writing-modes has some cases where orthogonal flows (vertical-in-horizontal or vice versa) ends up attempting to measure content, and failing because we haven't laid out that content yet, and falling back to 0.

thank you for the bug report, BTW!

but if you toggle on and off grid-area: side and grid-area: nav in the console it fix itself.

Yeah, I can confirm that this fixes the layout.

This is part of the known-issue; we get the wrong value (zero) the first time we query the orthogonal-flow for its content-size, and then we get a chance to lay out the content and discover its true size, and then we use the correct result the next time it's queried for a relayout.

(It's not really a complete/robust workaround, though; various things [e.g. closing devtools in this case] will cause Firefox to do a more-thorough relayout and invalidate the content measurements, which may make the content size be measured as 0 again.)