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.
Bug 1786087 Comment 4 Edit History
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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.)