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windows-ime-layer-transform.md

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Windows IME: layer transform for IMEOutput

Overview

FerriteEditor reports IME cursor geometry to the OS via egui::output::IMEOutput. Those rectangles must be in screen space (after the widget layer’s transform). This matches egui’s built-in TextEdit and fixes misaligned IME candidate windows on Windows when the editor lives in scaled or transformed UI (e.g. custom title bar).

Key Files

  • src/editor/ferrite/editor.rs — Sets IMEOutput after painting cursors and preedit; applies layer TSTransform to rect and cursor_rect.

Implementation Details

egui 0.28.1 does not provide layer_transform_to_global() (that name does not exist in this release). Upstream TextEdit reads ui.memory(|m| m.layer_transforms.get(&ui.layer_id())), then uses transform * rect and transform * cursor_rect. Ferrite uses the same pattern so OS-level IME positioning stays consistent with egui’s coordinate system.

If candidate z-order or placement is still wrong after this, further work may require Win32-specific behavior outside egui’s portable output.

Dependencies Used

  • egui — LayerId transforms in Memory::layer_transforms, TSTransform × Rect, IMEOutput.

Usage

Test (Windows): Enable an IME (e.g. Microsoft Pinyin), compose in the raw editor; the candidate list should sit near the composition cursor. Toggle custom window decorations and light/dark themes as a smoke test.

References

  • Upstream: egui text_edit builder IME block (~lines 729–738 in egui 0.28.1).
  • Related: GitHub #103, #15; ROADMAP (Windows IME).