Affected versions

• Firefox 76.0
• Firefox 77.0a1

Affected platforms

• Windows 7 & 10 (x64)
• macOS 10.15.4
• Ubuntu 20.04 (X64)

Steps to reproduce

1. Launch Firefox
2. Access Logins and Passwords from the menu
3. Press refresh repedeatly or keep pressed F5

Expected result

• The Search Logins bar is fixed in page

Actual result

• The search Logins quickly extends and retracts when refreshing the page

Regression range

• Will search for a regression.

Additional notes

• Have the Firefox's window stretched at half of the screen, and the issue will be more noticeable.
• The issue can be seen in the attachment

Regression range

• Last good: 20190912215412
• First bad: 20190913214459

Push log: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=eff20680fd653d5a771ad82c7fc95476e40bd35b&tochange=aeb5ff5e6aa737b19ff6acea77ebde9cdf5c3db1

Because this bug's Severity is normal and has not been changed, and this bug's priority is -- (none,) indicating it has has not been previously triaged, the bug's Severity is being updated to -- (default, untriaged.)

Given the regressing patch, I'm going to move this to Firefox::about:logins for frontend investigation first.

Just a guess, but it seems like an artifact of the layout, where the "Manage Account" content gets inserted later, causing a reflow and thus the "jump" with the sudden shrink of the flex item containing the search bar.

Yes, Sean is correct. This is not a layout bug but an artifact of how the UI was developed. I'm not sure what we would want to do here since the <fxaccounts-button> element can grow/expand with size based on the text we display and the logged in state of the user. The search field is also flexible. Making one of the two fixed in size would fix this, or stacking them vertically so they don't both try to resize each other. Another choice would be to hide them initially (visibility:hidden) until we know what state to display then make them both visible at the same time but that has the potential to make us look slow or glitchy too.