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* 8671aef Notify Win32FlutterWindow of cursor updates (flutter/engine#23795)

* c8620c3 Implement delayed key event synthesis for Windows (flutter/engine#23524)

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* 9365230 Add support for IME-based text input on Windows (flutter/engine#23853)

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This updates the Win32 desktop embedder to support input method (abbreviated IM
or IME) composing regions.

In contrast to languages such as English, where keyboard input is
managed keystroke-by-keystroke, languages such as Japanese require a
multi-step input process wherein the user begins a composing sequence,
during which point their keystrokes are captured by a system input
method and converted into a text sequence. During composing, the user is
able to edit the composing range and manage the conversion from keyboard
input to text before eventually committing the text to the underlying
text input field.

To illustrate this, in Japanese, this sequence might look something like
the following:

1. User types 'k'. The character 'k' is added to the composing region.
   Typically, the text 'k' will be inserted inline into the underlying
   text field but the composing range will be highlighted in some manner,
   frequently with a highlight or underline.
2. User types 'a'. The composing range is replaced with the phonetic
   kana character 'か' (ka). The composing range continues to be
   highlighted.
3. User types 'k'. The character 'k' is appended to the composing
   range such that the highlighted text is now 'かｋ'
4. User types 'u'. The trailing 'k' is replaced with the phonetic kana
   character 'く' (ku) such that the composing range now reads 'かく'
   The composing range continues to be highlighted.
5. The user presses the space bar to convert the kana characters to
   kanji. The composing range is replaced with '書く' (kaku: to write).
6. The user presses the space bar again to show other conversions. The
   user's configured input method (for example, ibus) pops up a
   completions menu populated with alternatives such as 各 (kaku:
   every), 描く (kaku: to draw), 核 (kaku: pit of a fruit, nucleus), 角
   (kaku: angle), etc.
7. The user uses the arrow keys to navigate the completions menu and
   select the alternative to input. As they do, the inline composing
   region in the text field is updated. It continues to be highlighted
   or underlined.
8. The user hits enter to commit the composing region. The text is
   committed to the underlying text field and the visual highlighting is
   removed.
9. If the user presses another key, a new composing sequence begins.

If a selection is present when composing begins, it is preserved until
the first keypress of input is received, at which point the selection is
deleted. If a composing sequence is aborted before the first keypress,
the selection is preserved. Creating a new selection (with the mouse,
for example) aborts composing and the composing region is automatically
committed. A composing range and selection, both with an extent, are
not permitted to co-exist.

During composing, keyboard navigation via the arrow keys, or home and
end (or equivalent shortcuts) is restricted to the composing range, as
are deletions via backspace and the delete key. This patch adds two new
private convenience methods, `editing_range` and `text_range`. The
former returns the range for which editing is currently active -- the
composing range, if composing, otherwise the full range of the text. The
latter, returns a range from position 0 (inclusive) to `text_.length()`
exclusive.

Windows IME support revolves around two main UI windows: the composition window
and the candidate window. The composition window is a system window overlaid
within the current window bounds which renders the composing string. Flutter
already renders this string itself, so we request that this window be hidden.
The candidate window is a system-rendered dropdown that displays all possible
conversions for the text in the composing region.  Since the contents of this
window are specific to the particular IME in use, and because the user may have
installed one or more third-party IMEs, Flutter does not attempt to render this
as a widget itself, but rather delegates to the system-rendered window.

The lifecycle of IME composing begins follows the following event order:
1. WM_IME_SETCONTEXT: on window creation this event is received. We strip the
   ISC_SHOWUICOMPOSITIONWINDOW bit from the event lparam before passing it to
   DefWindowProc() in order to hide the composition window, which Flutter
   already renders itself.
2. WM_IME_STARTCOMPOSITION: triggered whenever the user begins inputting new
   text. We use this event to set Flutter's TextInputModel into composing mode.
3. WM_IME_COMPOSITION: triggered on each keypress as the user adds, replaces,
   or deletes text in the composing region, navigates with their cursor within
   the composing region, or selects a new conversion candidate from the
   candidates list.
4. WM_IME_ENDCOMPOSITION: triggered when the user has finished editing the text
   in the composing region and decides to commit or abort the composition.

Additionally, the following IME-related events are emitted but not yet handled:
* WM_INPUTLANGCHANGE: triggered whenever the user selects a new language using
  the system language selection menu. Since there some language-specific
  behaviours to IMEs, we may want to make use of this in the future.
* WM_IME_NOTIFY: triggered to notify of various status events such as opening
  or closing the candidate window, setting the conversion mode, etc. None of
  these are relevant to Flutter at the moment.
* WM_IME_REQUEST: triggered to notify of various commands/requests such as
  triggering reconversion of text, which should begin composition mode, insert
  the selected text into the composing region, and allow the user to select new
  alternative candidates for the text in question before re-committing their
  new selection. This patch doesn't support this feature, but it's an important
  feature that we should support in future.