Fix embedded webview geometry on scaled displays (PR #63 + review follow-ups)

[shannah]

This branch starts from @Aunali321's commit in #63 (6cbe02d, kept as-is) and adds one commit that addresses the points raised in the review of that PR.

Included from #63

• Windows (heavyweight): webview_embed_set_bounds sizes the child HWND and the WebView2 controller from the canvas HWND's client rect instead of from AWT user-space w, h.
• Linux (lightweight): the hidden popup's -32000 offset is divided by the window scale, so it no longer wraps on screen at 2x.

Follow-ups (this branch)

Windows (Canvas 6 §6 + new operation 17)

• fit_child_to_parent is now the one resize routine. It zero-initialises the RECT and returns early if GetClientRect on the canvas HWND fails, so a late dispatched op can no longer push garbage bounds.
• The child now follows the canvas HWND's device size even when Java fires no componentResized. Example: the frame is dragged to a monitor with a different scale, so the canvas's device size changes but its user-space size doesn't. A thread-specific WH_CALLWNDPROCRET hook on the AWT toolkit thread watches the canvas's WM_SIZE and WM_WINDOWPOSCHANGED (size changed) and posts WM_EMBED_FIT_PARENT to our WebViewEmbedChild. The hook is refcounted per thread, acquired in create_engine and adopt_retained_popup, and released in destroy_engine. AWT's window procedure is never replaced, and the hook only posts. It never calls SendMessage, so the AWT thread can't block on the worker.

Linux lightweight (Canvas 33 D2)

• gtk_off_park_offscreen now does the scale-aware move. It runs once before the popup is shown, and again from a notify::scale-factor handler, so a runtime change to Gdk/WindowScaling re-parks the popup off screen.

SPDD

• Canvas 6 §6 constraint, new operation 17 and a Safeguard; Canvas 33 D2, operation 1 and Safeguard 1. Done via /spdd-prompt-update → /spdd-generate.
• The media analysis's "-32000 is off every monitor" rationale now accounts for scale. The story's mention is a historical repro at 1x and is left unchanged.

Verification

• Linux lightweight: xvfb-run ./run-linux-media-smoketest.sh passes all checks (A, B, P). With GDK_SCALE=2, xwininfo shows the popup at +-32000+-32000 device px. The runtime scale-change path was not exercised: it needs a live X settings daemon.
• Windows: not built or run here. The new hook, fit and refcount functions compile under MinGW with -Wall -Wextra against real windows.h. The full file needs MSVC/WRL, so CI has to confirm it. Needs on-device check: at a scale above 100%, drag the frame between monitors of different scale and confirm the page still fills the canvas without resizing the frame.

🤖 Generated with Claude Code

https://claude.ai/code/session_01HKYwyMrdMwg5rJNj38hxvj

---

Generated by Claude Code