Bug 251061

Created attachment 464625 [details]
How this issue happens in Safari.

// This issue might be related to issue 188370 but I don't know whether this is actually a duplication.

[macOS] Please stop websites handling Tab key events if it is already handled by an input method.

Safari version: 18614.3.7.1.5 (running 259258@main through Terminal.)

## Steps:

1. Install either vChewing (at least 3.3.1) or McBopomofo (at least 2.4.2) on macOS (at least 10.13).
2. Loggin to Facebook, use any of the input methods mentioned above to start typing something in a post editor, supposing that you are writing a new post.
3. Press (Shift+)Tab key to rotate between candidates.

## Description:
* The problem doesn't happen in FireFox.

By pressing the Tab key when the composition-buffer (whether inline or not, i.e. pre-edit area) is not empty, vChewing and McBopomofo rotates between candidates.

// vChewing shows the current candidate index (as a little popup tooltip) when rotating candidates.
// vChewing also supports inline candidate rotation with Option+Up/Down (or Option+Left/Right when vertical typing), mitigating the issue reported by this thread.

However, in the scenario mentioned in the "Steps to reproduce problem", both Chrome and Safari handles the Tab key even if it is "handled and returned true" by the input method.

It worths mentioning that any NSEvent handled and returned-true by an input method is not expected to be handled again by the client app. Both Safari and Chrome dismissed that.

(I'll file a separate bug report to Safari bugzilla.)

## Screenshot (video):

The following video demonstrate how Safari bugs with this (with different behavior):

https://user-images.githubusercontent.com/3164826/214209344-99a7e217-25e7-43d3-8384-32c7b03cc83c.mp4

This video demonstrates this issue using FireFox, Edge-Chromium and Chrome, indicating that FireFox doesn't have this issue.

https://user-images.githubusercontent.com/3164826/214208503-65f47b9d-bbc9-4ec0-a59e-e55b15dfffac.mp4

## Also:

The latest vChewing release can be fetched here: https://github.com/vChewing/vChewing-macOS/releases/

Ref: https://github.com/vChewing/vChewing-macOS/discussions/326