Some source designations are truncated, bleed into placeholder text

[eloquence]

Tested in 0.4.0-dev-20210204-154420 on Qubes

STR

1. Connect to a SecureDrop server with a significant number of sources (~50) so you see a large variety of source designations.
2. Cycle through the sources and observe the reply placeholder text ("Compose a reply to ..")

Expected behavior

Placeholders are displayed in full, or elided as implemented in #1145

Actual behavior

Placeholders are truncated mid-word, and sometimes bleed into the "Compose a reply" text.

Screenshot

This screenshot shows a designation that is both truncated on the right, and bleeds into the placeholder text on the left.

[eloquence]

Have so far not been able to reproduce on latest main in Ubuntu development environment, may not be visible outside Qubes. (I've tested it by assigning a very long source name in https://github.com/freedomofpress/securedrop-client/blob/main/securedrop_client/gui/widgets.py#L3533, which gets correctly elided on window resize).

[eloquence]

Not seeing this issue in Qubes anymore either, not on latest nightly nor on main. Closing for now until another repro.

[eloquence]

I'm seeing the truncation issue more reliably in offline->online mode transitions (only tested in Qubes for now):

1. Launch the app in offline mode
2. Resize the window to the smallest possible size
3. Select a source with a long name
4. Click "Sign in" and complete the login

The reply placeholder for the selected source is now cut off for me, without eliding. I don't believe this is a recent regression -- I see the same issue on the 0.4.0 tag as well.