User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:128.0) Gecko/20100101 Firefox/128.0
Summary
When a message in the Message List is active and selected (has been clicked on), the active message Card receives a nice noticeable highlight. But when the user clicks somewhere else, e.g. to select some "neutral" area in order to avoid accidentally doing something unwanted to the message (e.g. delete, archive, mark as junk) or into the message (e.g. to select text, scroll, click a link), then the Active message in the Message List loses almost all of its highlighting and thus becomes hard to locate in the Message List.
Theme in use: Mozilla's "System theme" in dark mode
Thunderbird Daily, 130.0a1 (2024-07-24) (64-bit)
OS: Windows 10
UI Description
Non-selected messages:
Pill background: slightly lighter dark-gray than surrounding background.
no other visual markings
Selected message:
Pill background: very pale version of OS's highlight color
Pill border: OS's highlight color
Thick vertical line to the left of the Pill: OS's highlight color
Active but not selected message (e.g. click into message text):
Thick vertical line to the left of the Pill: lighter dark-gray than non-selected messages' background
no other visual markings
NOTE: Very hard to distinguish from other messages.
Solution
Active but not selected messages uses the OS's highlight color for the thick vertical line to the left of the Pill.
Bug Report
I just filed bug 1909871 for this issue (https://bugzilla.mozilla.org/show_bug.cgi?id=1909871).
Screenshots
