Taskbar remains visible at the top when entering full screen in Firefox 138+

[offhub]

Describe what you noticed and did

1. Download Firefox 138+.
2. Install it in a sandbox.
3. Launch Firefox and press F11 to enter full-screen mode.
4. The taskbar remains visible at the top.

sbie4761fstb01.mp4

How often did you encounter it so far?

99.987%

Expected behavior

When pressing F11 or entering full-screen mode in Firefox 138+, the taskbar should automatically hide, allowing the browser to take up the entire screen without any obstructions. (like in Firefox 137)

Affected program

Firefox 138+

Download link

https://ftp.mozilla.org/pub/firefox/releases/138.0/win64/en-US/

Where is the program located?

Not relevant to my request.

Did the program or any related process close unexpectedly?

No, not at all.

Crash dump

No response

What version of Sandboxie are you running now?

Sandboxie Plus v1.15.11 64-bit

Is it a new installation of Sandboxie?

I recently did a new clean installation.

Is it a regression from previous versions?

No response

In which sandbox type you have this problem?

In a standard isolation sandbox (yellow sandbox icon).

Can you reproduce this problem on a new empty sandbox?

I can confirm it also on a new empty sandbox.

What is your Windows edition and version?

Windows 10/11

In which Windows account you have this problem?

A Microsoft account (Administrator)., A local account (Administrator).

Please mention any installed security software

MSD

Did you previously enable some security policy settings outside Sandboxie?

No response

Trace log

No response

Sandboxie.ini configuration

[wzxjohn]

Same on my site. The first fullscreen on 138 can hide the taskbar, but then it never hide again.

[ElHanko]

The same for me. The taskbar disappears on the first full screen, but not afterwards.

[DavidXanatos]

Does it work with 137.
that is did a ff change break it or a sandboxie change?

[offhub]

Does it work with 137.

Yes, it does.

that is did a ff change break it or a sandboxie change?

The issue began after updating to Firefox 138. Same thing occurs on older versions of Sandboxie,

[wzxjohn]

After reading the code and testing it, it is basically confirmed that the problem was introduced by this commit: mozilla-firefox/firefox@c82fa4c
This commit changes Firefox's Fullscreen's default logic by adding the use of NonRudeHWND API call.

Firefox provides an option to control the behavior here, which can be tested this way:

1. Open Firefox 138+ in Sandbox
2. Open the about:config page in Firefox
3. Search for the widget.windows.fullscreen_marking_method configuration entry

• This defaults to 0, which uses both the old and new APIs.
• If it's set to 2, which uses only the original MarkFullscreenWindow method, it doesn't trigger the bug.
• If it's set to 1, which uses only the NonRudeHWND method, it triggers the bug.

So I think this bug is caused by Firefox's newly introduced NonRudeHWND method.

Notice: First fullscreen after the config change may still have problem. Subsequent ones will be as expected.