E-mail headers are transparent and overlapping

### What Happened?

As demonstrated in the screenshot, clicking on the email headers in a conversation thread ends up in a situation where the e-mail header area (from/to/subject) has no background and overlaps with the widgets underneath.

### Steps to Reproduce

1. clicked on thread with 2 e-mails in the inbox
2. two e-mails are shown as stacked boxes
3. click one, then the other e-mail to expand it (maybe back and forth another time)
4. pretty quickly this ends up in the state shown in the screenshot

### Expected Behavior

E-mail that is shown in front should be fully opaque so that it doesn't overlap with the background widgets.

### OS Version

8.x (Circe)

### OS Architecture

amd64 (on most hardwares)

### Session Type

Secure Session (Wayland, This is the default)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell

```

### Hardware Info

HP ProDesk mini PC with Intel onboard graphics