1734189 - Restructure the message compose header area

Status: NEW

(Thunderbird :: Message Compose Window, enhancement)

Description

[Alessandro Castellani [:aleca]]

The message compose header area went through a series of updates with the pills implementation, focus ring improvements, and so on, bringing great improvements to the overall experience.

There area some areas which are still problematic and are a symptom of the underlying structure and styling we're currently using.
The most glaring issues are:

• Automatic resizing the header area is a bit wonky, with the scrollbar kicking in also in situation where it's not necessary. bug 1720348
• When opening the compose window with the BCC and CC fields visible by default, the header is already resized with an overflow even when not necessary.
• We're still forced to manually write the sizing of the labels area (From, To, ect.).

Improvements proposals:

• Use CSS grid for the layout of the whole area, removing the necessity of hard coding the sizing of the labels area.
• Explore a smarter way to handle many addresses rather than just adding a scrollbar (smart collapse of multiple pills, better height detection of the entire compose window, etc.)

Doing so will also give us easier control to implement sorting and ordering of pills which will drastically improve the usability of this area when dealing with many addresses (bug 1635207).