Configuring preferences and options in Outlook.com
@Ebuck said
when I compose or reply to a message, my top reading pane disappears
Reading Pane during Compose or Reply is bugging out for may people.
Does yours look like a variation of what is shown in the Answer by "Hornblower409 Jul 28, 2026" at
https://learn.microsoft.com/en-us/answers/questions/5958408/gro-e-fl-che-ber-neuer-nachricht-im-outlook
Try changing your Reading Pane to Right or Fill. Any difference?