FormCanUndo method

FormCanUndo wraps PDFium's FORM_CanUndo. It returns True when the FOCUSED AcroForm widget has at least one edit in its undo history that FormUndo could roll back, and False otherwise

Returns False when the view is inactive, the document has no AcroForm or no widget is focused — matching the contract of the other five form-edit helpers

The natural use is as the Enabled-state expression for an Undo menu item, toolbar button or context-menu entry

• FormCanUndo is per-widget — the answer changes as the user shifts focus between widgets, since each widget owns its own undo stack
• Call FormCanUndo from an OnUpdate / OnIdle / menu-popup handler if the UI surfaces an Undo button so the enabled state stays in sync with PDFium's view of the world
• FormCanUndo is a cheap probe — no document mutation, no JavaScript fire