TINYDOC-3435 - Document undo level grouping and IME typing behavior

[kemister85]

Ticket: TINYDOC-3435

Site: Staging

Changes

Closes a documentation gap where users could not understand why a single Ctrl+Z can undo a large block of typed text — most visibly when typing Chinese, Japanese, or Korean via an Input Method Editor (IME).

Added two subsections to the custom_undo_redo_levels option (modules/ROOT/partials/configuration/custom_undo_redo_levels.adoc, rendered on the Content behavior options page):

• How typing is grouped into undo levels — explains that consecutive typing is merged into one undo level until a boundary event (navigation keys, clicking, blur, or an executed command) closes it; that this is input-method independent but more noticeable with IME input where a whole composed phrase can become one level; and that custom_undo_redo_levels controls stack depth, not grouping granularity, with no built-in per-keystroke option.
• Creating finer-grained undo levels — documents the supported workaround of calling UndoManager.add() at chosen checkpoints, with a timer-based example, and cross-links the UndoManager API.

Validation

• Behavior verified against tinymce source (core/main/ts/undo/Setup.ts, TypingState.ts): typing coalescing, boundary events, and the absence of composition-event handling.
• Workaround corrected against source — blur and SaveContent already create undo levels automatically (Setup.ts:60), so the example uses a timer, which is the accurate way to add finer-grained checkpoints during continuous typing.

Prepared for documentation team review.