(In reply to Josiah Bruner [:JosiahOne] (needinfo > CC) from comment #109)
> Created attachment 8544150
> Example sketch.

I'm not sure if the sketch UI will work out well as presented; imo it might easily create confusion and wrong assumptions about this feature.

From selector in compose UI of current release has simultaneous double function:
Select a predefined sender identity as a set consisting of
a) From-field of sent msg: John Doe <john(at)gmail.com>
b) SMTP-Server & login for sending: smtp.gmail.com, port: 465, SSL/TLS, normal password, username: john(at)gmail.com
If I understand this bug correctly, for a given(!) identity selected by user from the dropdown, we want to allow editing a) (From-field) only, but we still rely on getting b) SMTP-Server from the original identity.

"Enter a custom address...", more so at the bottom(!) of the dropdown list, wrongly suggests that user could create a new address/identity at the same level as the other identities in the list, which is not the case. Instead, the UI should reflect that we only allow window-dressing of the From-field text value within a predefined identity (which btw will not work for many commonly used mail servers, so we also need to think in terms of ux-error-prevention!).

I agree with Josiah's comment 102 (supported by Ben's comment 107):

(In reply to Josiah Bruner [:JosiahOne] (needinfo > CC) from comment #102)
> Comment on attachment 8507858
> 87987.png
> 
> I think what we should do is ... show the drop down if they
> click anywhere other than an edit button. The edit button could go where the
> dropdown icon is now for example.

****************************
Tentative UI/UX proposal:
****************************

1) non-hover, non-focus:
[John <john(at)gmail.com>                             v]

- hide edit icon/button
- single-left-click from-selector anywhere opens dropdown (big click target for default workflow of picking predefined identity from list, as proposed by Josiah/Ben)
- double-click anywhere -> edit sender (double click does nothing useful otherwise, so it's free).

2) hover, focus:
[John <john(at)gmail.com>                         [/] v]

- only when hovered or focused, show a small "Edit" icon [/] (e.g. pen)
- imo per ux-natural-mapping, the correct position for this icon, as hinted by Josiah, is *inside* the dropdown (correctly implying you can edit the inner value based on prior choice in dropdown).
- clicking the "Edit" icon allows editing of From field value, either dialogue (allowing explanation of dangers involved for unsuspecting users), or inline (how do we handle ux-error-prevention then?)

3) popup (without editing):
[John <john(at)gmail.com>                         [/] v]
|John private  <john.private(at)foo.bar>               |
|John tertiary <john.tertiary(at)asdf.com>             |
+------------------------------------------------------+

- if user hasn't edited anything, clicking anywhere on from-selector outside "Edit" icon will open the normal and unmodified popup
- imo we should *not* advertise this feature more by offering a dedicated action row inside the dropdown, ux-error-prevention for default users: many smtp servers will reject altered from-fields, especially when you change the email address
- consider if the entire feature should be preffed-off by default, again ux-error-prevention

4) popup (with/after editing):
[Johnny Edited <john(at)fakemail.com>             [/] v]
|*John <john(at)gmail.com>                             |
|John private  <john.private(at)foo.bar>               |
|John tertiary <john.tertiary(at)asdf.com>             |
+------------------------------------------------------+

- after manual editing, we need to offer an easy way of reverting to the original identity whose from-sender was edited: offer as first entry in the dropdown, with some indication (icon, bold, etc.) that this the currently used identity (only with edited from).

5) Context menu
+--------------------+
|Edit this sender    |
|Copy                |
|Paste               |
+--------------------+

- optionally for this bug, but recommended, implement a simple context menu on the from-selector
- Edit this sender -> edit sender (dialog/inline)
- Copy: Copy the current value of sender, either original  "John <john(at)gmail.com>" or edited "Johnny Edited <john(at)fakemail.com>"
- Paste: Paste clipboard text to become the new edited value of sender
- Copy and Paste might need better strings to indicate what they do