feat(ui): a middle click closes a tab (#1853)

[mtlcl410]

What this changes

Fixes #1853. A middle click on a tab closes it, as in Acrobat and browsers.

• A document tab goes through the same path as its close button (close = Some(i) → Action::Close → request_close_tab), so a document with unsaved changes asks first instead of closing.
• The Combine files tab closes on a middle click too.
• Nothing else changes: a left click still activates the tab, and the context menu is as before.

Middle-button auto-scroll (autoscroll.rs) only starts inside the document viewport, so it doesn't see a click on the tab strip. Response::middle_clicked is egui's own per-button click tracking on the tab's existing click sense.

How you verified it

• cargo fmt --check
• cargo clippy -p pdfcraft-ui-egui -j 2 --all-targets -- -D warnings
• cargo test -p pdfcraft-ui-egui -j 2 --test ui: 45 of 46 pass. The one failure, dropping_a_pdf_on_the_window_opens_it, fails on main on Windows without this change too: it runs a fixed three frames after the drop while the background open worker (Desktop: prepare large PDFs off the UI thread #1493) is still preparing the document. test(ui): extend native opening and continuation regression coverage #1764 already fixes it with a wait_for.

Test. a_middle_click_closes_the_tab (kittest) opens two documents, middle-clicks the first tab and requires that only the second remains, with no close prompt. It then edits the remaining document, middle-clicks its tab and requires that it is still open and a close request is pending.

With chrome.rs reverted the test fails: left: 2, right: 1 (both tabs still open).

Never-crash

• No new unwrap/expect/indexing outside tests
• No unsafe

Provenance

• No assets, no external code. The behaviour is the common tab convention the issue describes.

Security

• No paths, MCP, control channel or network involved

Parity and reach

• A small interface convention; no parity/ entry describes tab gestures, so none changes. Not a headless operation, so no automation tool.

🤖 Generated with Claude Code

[echelon]

Thank you @mtlcl410 for closing tabs with a middle click! It's merged into main now.

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