Description of Change

Fixes #51731.

Calling win.center() caused frameless windows with insets to shrink on both Windows and Linux. Additionally, on Windows, frameless windows were not centered correctly on launch or when calling win.center().

This PR corrects both the shrinking and the positioning by introducing a shared implementation for NativeWindowViews::Center() for Linux and Windows. It is based on the existing Linux codepath but has been updated to use SetBounds(), which properly handles insets, instead of the lower level widget()->SetBounds(). On Windows, it replaces the call to gfx::CenterAndSizeWindow(size), which places windows too high if they have asymmetric insets.

While CenterAndSizeWindow could be patched upstream (this is an approach I have taken for other APIs) I looked at its implementation and do not see a benefit in having a divergent path for Windows in this instance. The upstream API is not cross-platform and is not used much even on Windows, and Electron isn't getting anything for free that it doesn't already have to support to position windows correctly. It is possible that I'm missing something, but I have done my best to test this simplification thoroughly.

With the new shared implementation, centering on both platforms is now accurate (at least at 100% DPI) and does not change the window's size. I was also able to get rid of an extra call to set bounds on init now that Center() has an improved implementation.

I added new tests for both initial window bounds and win.center() for framed and frameless windows. I also verified that frameless and framed windows are centered in the exact size and position on Windows as in 41.2.0 (prior to the insets changes).

(On Wayland, win.center() is still effectively a no-op, as the compositor will not honour the move. However, the window will no longer shrink if it is called.)

Backports to 41, 42, 43.

Checklist

• I have built and tested this change
• I have filled out the PR description
• I have reviewed and verified the changes
• npm test passes
• tests are changed or added
• relevant API documentation, tutorials, and examples are updated and follow the documentation style guide
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Fixed an issue on Linux and Windows where frameless windows would shrink when calling win.center(). Additionally, fixed an issue where frameless windows would appear slightly off-center on Windows.