Unexpected Gray Border and Incorrect Width in Fullscreen Mode with DevTools Open in Edge

[charliefr]

Describe the bug
When Microsoft Edge is in fullscreen mode with DevTools open, an unexpected gray border of approximately 4px appears on the left, right, and bottom sides of the screen. The corners of the border are rounded. This issue is observed on a 4K monitor with a system scale of 150%.

To Reproduce
Steps to reproduce the behavior:

1. Open Microsoft Edge on a 4K monitor with system scale set to 150%.
2. Enter fullscreen mode.
3. Open DevTools.
4. Run document.body.getBoundingClientRect() in the console.
5. Observe that the returned width is 2552px, which is less than the expected 2560px (3840px physical width / 1.5 scale factor).

Expected behavior
In fullscreen mode, the width returned by document.body.getBoundingClientRect() should be 2560px, and there should be no gray border visible on the screen.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. Windows 10] 19045.4529
• Browser: Microsoft Edge
• Version: 126.0.2592.61 x64

Additional context
This issue only occurs when DevTools is open in fullscreen mode. When DevTools is closed, the fullscreen display works as expected.

AB#51910494

[captainbrosset]

Thank you for filing, I was able to reproduce. Here is a full screen screenshot:

And a close up of the border, near the bottom-left corner: