fix: selection toolbar/handles track scroll viewport like TextField

[RekanDev]

Summary

• Use the scroll viewport (not the full document size) for selection-handle visibility so handles fade when scrolled out of view, matching multiline TextField / RenderEditable.
• Hide the Cut/Copy/Paste context menu while scrolling and restore it when scrolling ends if the selection is still on-screen (including after scrolling away and back).
• Clamp context-menu anchors into the visible editor region when the selection extends past the viewport.
• Show the context menu again after selection-handle drag ends (or a handle tap), matching TextSelectionOverlay behavior.

Test plan

• On Android/iOS: select text in a scrollable QuillEditor, verify Cut/Copy/Paste appears
• Scroll down with an active selection: menu hides while scrolling; restores if selection remains visible
• Scroll up so selection leaves the viewport: menu stays hidden; scroll back until selection is visible and release — menu restores without re-selecting
• Selection handles should not float outside the editor when scrolling; they should fade when off-screen
• Drag selection handles: after release, context menu appears again
• Compare with a multiline TextField for similar feel

Made with Cursor