set_outer_position and request_inner_size use current monitor's scale factor instead of target monitor's on macOS

[natepiano]

Description

Description

When calling set_outer_position() or request_inner_size() to move/resize a window to a position on a different monitor with a different scale factor, winit converts the coordinates using the current window's monitor scale factor instead of the target monitor's scale factor.

This is the write-side counterpart to #2645, which documents the same fundamental issue for read operations (outer_position(), MonitorHandle::position()).

Environment

• macOS (tested on Tahoe 26.1)
• Multi-monitor setup with different scale factors (e.g., Retina 2.0x + external 1.0x)

Reproduction

1. Connect a Retina display (scale factor 2.0) and an external display (scale factor 1.0)
2. Launch application on the 1.0x monitor
3. Save a window position from the 2.0x monitor (e.g., physical position (1728, 68))
4. Call set_outer_position(PhysicalPosition::new(1728, 68)) to restore

Expected: Window appears at physical position (1728, 68) on the 2.0x monitor

Actual: Window appears at physical position (3456, 136) — coordinates are doubled

Root Cause

In the macOS backend, set_outer_position converts PhysicalPosition to the internal coordinate system using backingScaleFactor() from the window's current monitor. When the target position is on a different monitor, this conversion is incorrect.

Similarly, request_inner_size uses the current monitor's scale factor when converting physical size to logical size for setContentSize:.

Impact

Applications that save/restore window position across sessions cannot correctly restore windows to monitors with different scale factors than the launch monitor. The position/size error is proportional to the scale factor ratio between monitors.

Workaround

We currently compensate by pre-multiplying coordinates by (current_scale / target_scale) before calling these methods, but this requires:

1. Knowing which monitor the window will land on
2. Different strategies for high→low vs low→high DPI transitions
3. A two-phase approach for high→low transitions due to size clamping

Related Issues

• MonitorHandle::position and MonitorHandle::size metrics are unclear  #2645 — Same root cause, but for read operations (outer_position(), MonitorHandle::position())
• HiDPI scaling finetuning #837 — HiDPI scaling architecture discussion

macOS version

ProductName:		macOS
ProductVersion:		26.1
BuildVersion:		25B78

Winit version

0.30