cosmetics: fixed arrows in thread view while scrolling down

[yeupou]

Hello,

Describe the bug
In thread view, with Firefox and Chromium, the arrows in front of each message in the thread are fixed with the default theme.
It break the layout as soon as we scroll down

Expected behavior
The should not be fixed.

Screenshots
Obviously, it is noticeable on long threads (otherwise no scrolling down or up). For the demo purpose in just reduce the window size:

Please complete the following information:

• Browser: chromium 109, firefox 109
• SnappyMail Version: 2.25.3

[the-djmaze]

Already solved in commit a0a9d8f