Problem statement
We are aware of an issue that seems to have begun on September 3, 2024, where in SharePoint Online, Microsoft has switched custom lists to a new user experience, which is causing custom forms you have created with Lightning Forms to open in “full screen” mode rather than in a side panel.
October 4 update: Microsoft reverted the above UI update on September 6 to address issues with the new UI, but they have now rolled the UI update out again - currently in Targeted Release tenants.
Investigation
In the new list UI that Microsoft rolled out around September 3 and 4, we see that uncustomized SharePoint forms (i.e. the default NewForm, DispForm, and EditForm) now open in a modal dialog (a popup in the center of the screen). We suspect that this change has caused customized forms to open in a full-screen mode in the same browser tab. We note that subsequently opening a sublist form (for example to create a new sublist item) does open in a side panel.
It seems that older custom lists might not be affected by this change, but this is perhaps only a temporary situation; the new UI might continue to roll out to all SharePoint lists.
Others have reported their customized forms (using other customization methods) opening in a new browser tab, so this is likely also related. See, for example, Forms-customizer opens in a new TAB · Issue #9904 · SharePoint/sp-dev-docs (github.com)
Sept 5: We are continuing to investigate, and to work with Microsoft on this issue.
Oct 4: Lightning Forms developers are working to address this, now that Microsoft has pushed the new UI back out again to Targeted Release tenants.
Resolution
As of September 6, Microsoft rolled back this update, so customized forms were opening in a side panel again. Our developers are investigating how to handle this if Microsoft pushes out this update again, assuming that they do indeed plan to default to opening forms in a modal dialog. We hope that if Microsoft does make this change again, it will be to early release tenants first, rather than to everyone as they did this time.
As of October 4, this issue has recurred, as Microsoft has begun rolling out the new List UI again, and we are working to address it as quickly as possible.