[fix] TextInput onContentSizeChange reports a shrinking content size

[u9g]

onContentSizeChange on a multiline TextInput reads the textarea's own scrollHeight, which is never less than the height the textarea currently has. Once an auto-growing input has grown (the app sets height from the reported size), deleting lines keeps reporting the old, larger size, so the input can never shrink back. This is #2160 (and the earlier #1339/#1399).

The fix measures a zero-height, hidden copy of the textarea instead of the textarea itself. The copy is a cloneNode() (so it keeps the same classes and inline styles, and inherits from the same parent), gets the textarea's computed width so wrapping matches, and is position: fixed / visibility: hidden / overflow: hidden so it neither moves anything nor grows a scrollbar that would change wrapping. It is inserted and removed synchronously, so nothing is painted. This is the same technique react-textarea-autosize uses. Temporarily setting the real textarea's height to 0 would be simpler, but the forced layout clamps the scroll position of any container the textarea sits at the bottom of (a chat composer being the typical case), and restoring the height does not restore the scroll offset.

Reported size is still scrollHeight/scrollWidth of a textarea (content plus padding), so values for content that fits are unchanged; only content smaller than the box now reports its real height.

Tested with a jest test that mocks scrollHeight (jsdom has no layout) and checks the reported height goes 10 → 30 → 10 as lines are added and removed, that the real textarea's height is untouched and that no clone is left in the DOM. Also checked in Chrome on a page scrolled to the bottom: a 100px-high textarea with one line reports 40px (line + padding) instead of 98px, eight lines report 180px as before, long lines wrap to the same height as in the textarea, and window.scrollY is unchanged after measuring. npm run unit, lint and format pass for the package; flow could not be run locally (flow-bin has no arm64 build), so relying on CI for it.

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