1881485 - Show an error icon for idref/idreflist attributes where referenced element can't be found

Status: NEW

(DevTools :: Inspector, enhancement, P3)

Description

[Nicolas Chevobbe [:nchevobbe]]

This would help detect typos or cases where the referenced element is no longer/not yet in the page
Note that we do emit an event on the client where the "Select element" button is clicked but we couldn't find it (see https://searchfox.org/mozilla-central/rev/a8cc31504a2379bcf8ba395d2da7bb632b5521d6/devtools/client/inspector/markup/markup.js#1218-1219,1223-1224,1226-1227), but this requires the user to actually click on it.