Affected versions

• Firefox 65.0 (20190124174741)
• Firefox 66.0.3 (20190409155332)
• Firefox 67.0b11 (20190415085659)
• Firefox 68.0a1 (20190415213626)

Affected platforms

• macOS 10.12
• Windows 10 x64
• Ubuntu 18.04 x64

Steps to reproduce

1. Open Firefox with a new profile and resize it to a quarter of the screen.
2. Go to about:preferences#privacy and click on “Manage Exceptions” button.
3. Resize the opened pop up window making it bigger and bring it back to initial dimensions.
4. Close the window.
5. Press again on the “Manage Exceptions” button.

Expected result

• The opened window has the same style allowing the user to scroll when the browser window height is constrained.

Actual result

• The opened window height is larger than the browser window.

Regression range

• Last good revision: f0f1aaf051d6798e1e73d1feee07ca847333167a (2016-10-21)
• First bad revision: 5639a9f476d08f300c079117e61697f5026b6367 (2016-10-22)
• Pushlog:
  https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=f0f1aaf051d6798e1e73d1feee07ca847333167a&tochange=5639a9f476d08f300c079117e61697f5026b6367

Additional Notes

• Only enlarging the window don’t trigger the issuer, the issue is triggered when making the window smaller.
• On unaffected builds the behavior was different when resizing the window truncating the buttons but closing and reopening it would fix the issue.
• The issue is reproducible on almost all pop up windows from the about:preferences page.
• Vertically resizing the browser making it smaller than the time issue occurs resets the behaviour.
• Attached a screen recording with the issue.

Maybe related to bug 1287209? kmag, what do you think?

Seems pretty unlikely