Many weird behaviours with text selection

The _Rich Text_ demo have many weird behaviours with text selection when:

1. Triple-clicking
2. Dragging the mouse vertically
3. Shift+Up/Down
4. Resizing the viewport
5. Zooming the viewport

You can try all these 5 actions with the non-pretext paragraph at the top, then with the pretext-enabled paragraph below, and notice the inconsistencies. Also, compare all that to the **pdf.js** demo.

Links:

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Start with the Rich Text demo at chenglou.me/pretext/rich-note/ and reproduce selection using triple-clicking, vertical dragging, Shift+Up/Down, viewport resizing, and zooming for both paragraphs. Compare each result with the pdf.js demo; done means the non-pretext and pretext-enabled paragraphs behave consistently across these actions.

Written by the indexing model from the issue text.