A single off-screen TextBox holds focus while an imgui text field is active, so the OS
delivers typing (any layout/unicode), paste and copy/cut. Its text mirrors imgui selection
(fully selected) so Ctrl+C/X copy exactly that; deletions apply as an imgui Backspace;
Ctrl is released around pasted characters. Focus is captured after the click is released,
kept on clicks inside imgui, ends on clicks outside. KeyCode text kept as fallback.
SetTextProxyDebug for logging.
4DBug