[web] Fix other places that use clientX/Y rather than offsetX/Y (text input?, semantics helper?)

[ditman]

So far, these are the places that I've found that aren't using the correct coordinates for the events they're handling (they'd only work in full-screen mode):

pointer_binding.dart

In the pointer_binding, it seems that 'pointerup' events are incorrectly attached to domWindow. Those should be attached to the glassPane. See:

• https://github.com/flutter/engine/blob/main/lib/web_ui/lib/src/engine/pointer_binding.dart#L690

(There's probably a need to synthesize a 'pointercancel' event if there's a pointerup detected OUTSIDE of the glassPane too!)

text_field.dart

See:

• https://github.com/flutter/engine/blob/main/lib/web_ui/lib/src/engine/semantics/text_field.dart#L294-L299

(and below)

semantics_helper.dart

See:

• https://github.com/flutter/engine/blob/main/lib/web_ui/lib/src/engine/semantics/semantics_helper.dart#L331-L340

(Using client coordinates rather than offset. Note that for touch events, there's no "offset" and it needs to be computed from the clientBoundingRect of the event target!)

[ditman]

Some of the pointer binding TODOs link here. For those related to "not using the _globalTarget". For embedded mode, all events are already rooted in the flutter-view, but for full page we're still using "window".

Flutter should setPointerCapture() on pointerdown to the flutter view element, and let the browser do its thing, instead of listening from two different event sources. See Pointer events > pointer capture in MDN.