Fix text selection in mouse-tracking sessions on macOS

[Bono2007]

Problem

Text cannot be selected with the mouse on macOS in any session running a TUI that enables mouse tracking, which makes copying impossible.

When an application turns on mouse tracking (CSI ?1002h / CSI ?1003h), xterm.js forwards every drag to that application instead of starting a selection. Terminal.app and iTerm2 let you hold Option to override the convention, but xterm.js gates the same escape hatch behind macOptionClickForcesSelection, which defaults to false and was never set here.

The symptom users report is misleading. Cmd+C finds no selection, so xterm's copy handler is a no-op and it fails silently — the previous clipboard contents stay in place. The next paste then inserts stale text, which reads as "paste is broken" even though the paste path itself is fine.

Reproduction

Measured against the live renderer over the Chrome DevTools Protocol, driving synthetic mouse and key events. The session runs vim -c "set mouse=a" on a file containing one known line:

mouseTrackingMode = "drag"

[option OFF] plain drag  -> ""
[option OFF] Option+drag -> ""

[option ON]  plain drag  -> ""
[option ON]  Option+drag -> "the quick brown fox jumps over the lazy dog"

clipboard after Cmd+C    =  "the quick brown fox jumps over the lazy dog"

Same gesture, same session, only the option toggled — which isolates the cause. Plain drag correctly keeps going to the application in both states, so mouse-driven TUIs stay usable.

Fix

One option on the xterm instance in public/terminal-manager.js:

macOptionClickForcesSelection: true,

Windows and Linux were never affected — there shouldForceSelection keys off Shift, which needs no option.

Verification

• Full round trip in a mouse-tracking session: Option+drag → Cmd+C → Cmd+V returns the selected line intact.
• No regression in sessions without mouse tracking: plain drag, Option+drag, double-click (one word) and triple-click (full line) all behave as before.
• npm test passes, npm run bundle:codemirror succeeds.

Trade-off

Enabling this flag disables Option+drag rectangular (column) selection on macOS, since xterm's shouldColumnSelect and shouldForceSelection share the same modifier. This is the standard trade-off and matches iTerm2's default behaviour.

Related

Distinct from #54 / #55, which cover copy on Linux/Wayland via OSC 52. Worth noting separately that Switchboard doesn't handle OSC 52 at all, so an application's own clipboard requests are dropped — out of scope here.