[golba98]

Add UIContext-owned overlays and reusable modal dialogs, then build anchored transient popups and a non-editable ComboBox on the same ownership and dispatch mechanisms.

Changes

• Add context-specific overlay handles, modal focus boundaries and restoration, safe dismissal, stacked dialogs, and deferred completion callbacks.
• Add owner-scoped popup placement, scrolling, modal integration, gesture-release suppression, and ComboBox selection/highlight separation, disabled choices, and deterministic type-ahead.
• Include focused examples, headless and renderer regression tests, installed-package consumer coverage, CI smoke tests, and usage documentation.
• Add opt-in native presentation diagnostics and a minimal GLFW control. Production rendering remains event-driven; no repaint timers, extra swaps, or synchronization workarounds were introduced.

Validation

• Rebuilt Debug and Release and reran LIBGL_ALWAYS_SOFTWARE=1 xvfb-run -a ctest --preset debug / release: 9/9 suites passed in each.
• ASan/UBSan: popup, ComboBox, dialog, and UI suites passed (4/4).
• Installed consumers and all six examples passed in both presets, including launches from /tmp.
• Controlled native captures: all 15 ComboBox scenes matched between demand and continuous modes; demand mode settled to zero swaps during the final 0.5-second idle sample.
• Warning-enabled diagnostic compilation, Python byte-compilation, and git diff --check passed.

Presentation investigation and limits

The original intermittent all-black root capture was not reproduced and its cause remains unproven. Explicit GL front reads can be black while native captures are correct, and post-swap/idle reads can perturb resize presentation; the minimal GLFW control reproduces these effects. Live desktop client capture succeeds while root capture returns BadMatch. The report distinguishes those observations from a confirmed MyGUI defect.

Windows and macOS were not tested locally. Raw captures and logs remain in the ignored local build directory; the committed diagnostic tools and report provide reproduction commands.