Pasting multi-line content (e.g. plain-text-style emails) creates excessive spacing between lines, even with explicit margin:0

` or `

`) — which is how several email clients format a plain, non-nested message (Outlook desktop, Outlook Web/"New Outlook", Gmail, Apple Mail) — Trix converts every one of those blocks into its own separate Trix paragraph. Trix's own default margin between paragraphs then applies to each one, producing noticeably larger gaps between lines than the source had, even when the source explicitly sets `margin: 0` on those elements.

## Reproduction

```html

Line one

Line two

Line three

```

Paste this (or call `editor.insertHTML(...)` with it directly). Each line becomes its own block in Trix's document model, rendered with Trix's default block margin — the source's `margin:0` has no effect.

## Investigation notes

- Reproduced with a real-world sample: a multi-paragraph email copied from Outlook Web ("New Outlook"), several paragraphs carrying an explicit `style="margin: 0px"`. Same result.
- **Not a recent regression.** Tested this exact input against Trix 2.1.8 (before DOMPurify was added for paste sanitization in 2.1.9, per the [CVE-2024-53847](https://github.com/advisories/GHSA-6vx4-v2jw-qwqh) fix) and current 2.1.19, side by side, calling `editor.insertHTML()` directly with no other app code involved — output was byte-for-byte identical on both. So this isn't a DOMPurify sanitization side effect or tied to any particular version; every version checked behaves the same way.
- `getBlockElementMargin`/`window.getComputedStyle` machinery in the HTML parser (still present in 2.1.19) looks like it's meant to read a pasted element's own margin, but empirically doesn't prevent the splitting/spacing described above for a plain multi-`

` paste.

## Environment

- Trix 2.1.8 and 2.1.19 tested directly; current app runs 2.1.19
- Chrome (via Puppeteer / Chrome for Testing) — haven't checked Firefox/Safari

## Workaround

We worked around this in our app by detecting "flat" multi-line pastes (a sequence of sibling `

blocks become Trix paragraphs and define tests for the intended spacing behavior; the issue does not specify an accepted direct-fix behavior, so completion criteria need maintainer agreement.