[yay]

Notice the slow cursor drift:

Screen.Recording.2026-10-26.at.9.09.52.AM.mov

Dragging a window by its title bar accumulates rounding error: each frame adds the pointer delta to an already rounded position. Repeated half-pixel back-and-forth movements can move the window even when the pointer returns to the same position.

Retain the initial window pivot and add the total drag displacement, then apply the existing constraints and pixel rounding. Preserve the anchor while the drag is held, including when PointerGone temporarily removes coordinates; clear it when the gesture ends. The small Area change also prevents drag-anywhere from jumping to its starting point when displacement is unavailable.

#7709 fixed this class of drift in Area using a fixed drag origin. The separate title-bar path introduced by #8183 used incremental deltas again. The regression tests protect that guarantee across title-bar, default, and drag-anywhere modes, five pixel scales, and layout retries. They also cover repeated gestures, returning from constraints, pointer exit/re-entry, and release outside the viewport. The fractional-motion and constraint-return tests fail without the fix.

This PR also fixes #4441.

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Preview available at https://egui-pr-preview.github.io/pr/8775-codexfix-title-bar-drag-drift
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