Since hapic feedback depends on nsIWidget / nsWindow, it isn't good if this service is in hal now. If this feature depends on window (Android and UIkit), it is better to add a IPC method in PBrowser, instead of hal. Accessible Caret has nsPresShell, so it can access nsIWidget (PuppetWidget in child process) directly. Currently, GeckoView doesn't support multiple screens and multiple windows in multiple screens. At the future, Android might have multiple windows like desktop, the window that shows accessible caret doesn't assures focused window, such as a race condition, or browser side implementation. So I think that is better to add nsIWidget::PerformHapticFeedback.
Bug 2018858 Comment 4 Edit History
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Since hapic feedback depends on nsIWidget / nsWindow, it isn't good if this service is in hal now. If this feature depends on window (Android and UIkit), it is better to add a IPC method in PBrowser, instead of hal. Accessible Caret has nsPresShell, so it can access nsIWidget (PuppetWidget in child process) directly. Currently, GeckoView doesn't support multiple screens and multiple windows in multiple screens. At the future, Android might have multiple windows like desktop. Also, the window that shows accessible caret doesn't assures focused window, such as a race condition, or browser side implementation. So I think that is better to add nsIWidget::PerformHapticFeedback.