2004659 - Window top border shows when maximized (on Windows 11)

Status: RESOLVED DUPLICATE of bug 1993474

(Core :: Widget: Win32, defect)

Description

[fireferret.forum]

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:145.0) Gecko/20100101 Firefox/145.0

Steps to reproduce:

The Firefox application window is maximised and has focus.

OS: Windows 11 25H2. Dark theme.
In Windows Settings, Personalisation > Colours, I have "show accent colour on title bars and window borders" set to On. The accent colour is set to Manual, with a colour selected (e.g. Gold)

Also affects Firefox 146.0b9 (Developer Edition)

Actual results:

There's a top border showing, in the accent colour (no side or bottom borders).

Even if I change the "show accent colour on title bars and window borders" to Off, there still seems to be a top border, in black. (There seems to be space of a few pixels allocated to the top border.)

Expected results:

When the application window is maximised there should not be a window border (on any edge).

Comment 1

[BugBot [:suhaib / :marco]]

The Bugbug bot thinks this bug should belong to the 'Core::Widget: Win32' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

Comment 2

[fireferret.forum]

More information:

This seems to happen when the Firefox app window is maximised on an external display (external to my laptop).

• The external display is set as the "main" display ("make this my main display" in System > Display).
• "Extend these displays" is chosen.
• The resolutions of the external and internal displays are different.

When the app window is maximised on the internal (laptop's) display and has focus, there is no top border (as expected).

(Note that it's only Firefox that gets a top border - other apps seem to behave ok.)