Edge-tiling on ultrawide (5120x1440, 32:9) anchors the window to its center instead of the screen edge

[Le-Syl21]

Describe the bug

On a 32:9 ultrawide monitor (5120x1440), dragging a window to the left or right
screen edge resizes it correctly to half the screen width (2560px), but positions
it incorrectly: the window's horizontal center is placed on the screen edge
instead of its outer edge. As a result, roughly half of the tiled window ends up
off-screen.

This is a regression after upgrading from Ubuntu 24.04 LTS to 26.04 LTS. On 24.04
the window snapped flush against the screen edge as expected.

Steps to reproduce

1. Use a 5120x1440 (32:9) display.
2. Make sure "Enhanced Tiling" is enabled.
3. Drag any window (e.g. Firefox) to the left or right screen edge and release.

Expected behaviour

The half-width (2560px) window snaps flush against the screen edge:
left tile at x=0, right tile starting at x=2560.

Actual behaviour

The window is sized correctly to 2560px wide, but anchored by its center to the
screen edge. When snapping right, the window's left edge ends up around x=3840
instead of x=2560, leaving ~1280px of the window off-screen on the right. Same
issue mirrored when snapping left.

Workaround / diagnostic

Disabling the "Enhanced Tiling" extension fully restores correct behaviour:
GNOME's native edge-tiling anchors the window flush to the screen edge as
expected. The issue is therefore in this extension's placement logic, not in
Mutter.

System information

• Ubuntu version: Ubuntu 26.04 LTS (upgraded from 24.04 LTS)
• Session type: Wayland (26.04 is Wayland-only)
• GNOME Shell version: 50.1
• Extension: Ubuntu Tiling Assistant (tiling-assistant@ubuntu.com), version 54
  (bundled with GNOME Shell in 26.04, no separate deb package)
• Monitor: single 32:9 display, 5120x1440, display scaling at 100%
• Ubuntu default dock, left side, 48px icons