Fix pasting in terminals

[colin-grant-work]

What it does

Fixes paste (Cmd+V / Ctrl+V) in the integrated terminal, and along the way restores the documented effect of the terminal.enablePaste and terminal.enableCopy preferences.

Root cause. The global ctrlcmd+v keybinding for CommonCommands.PASTE (see common-frontend-contribution.ts:841, enabled in Electron) intercepts the keystroke at the document level before xterm.js's hidden textarea can fire its native paste event. xterm's own paste pipeline — which would call Terminal.paste() and forward to the PTY, including bracketed-paste sequences — never runs. The same dynamic made terminal.enablePaste / terminal.enableCopy effectively dead, since they were only checked by TerminalWidgetImpl.customKeyHandler, which the global keybinding preempts.

Approach. Follow VSCode's terminal-clipboard pattern, using the keybinding priority mechanism from #16763:

• Register terminal-specific commands workbench.action.terminal.paste and workbench.action.terminal.copySelection, each with its own ctrlcmd+v / ctrlcmd+c keybinding scoped when: 'terminalFocus'.
• Each TerminalWidgetImpl declares terminalFocus as a local context key on its own DOM scope (contextKeyService.createScoped(this.node).createKey('terminalFocus', true)). When focus is inside a terminal, KeybindingRegistry.selectBindingByLocalContext prefers the terminal-scoped binding over the unconstrained global CommonCommands.PASTE / CommonCommands.COPY.
• Paste routes through a new TerminalWidget.paste(text) method that delegates to xterm's Terminal.paste() (preserving bracketed-paste handling). Copy keeps using TerminalCopyOnSelectionHandler.syncCopy().
• Handlers honor terminal.enablePaste / terminal.enableCopy. When a preference is off, the keybinding's isEnabled returns false and falls through to the global binding — matching the literal reading of the preferences' descriptions.

Outcomes.

• Cmd+V / Ctrl+V and Cmd+C / Ctrl+C work in the terminal (keystroke, terminal context menu).
• terminal.enablePaste / terminal.enableCopy regain their effect on the keystroke and context-menu entries; no effect on editor / Command Palette / input-field paste-copy.
• CommonCommands.PASTE and CommonCommands.COPY are no longer overridden by the terminal package. The registerHandler(CommonCommands.COPY.id, …) from fix(terminal): copy via context menu does not work #17290 is removed; the user-visible fix it landed (context-menu Copy) is preserved by the new structure.

How to test

1. Open Theia (esp. macOS Electron) and a terminal.
2. Paste: copy text, then in the terminal use Cmd+V / Ctrl+V and right-click → Paste; both should insert at the prompt.
3. Bracketed paste: paste a multi-line snippet into bash/zsh/python REPL; lines should not be executed individually or auto-indented.
4. Copy: select text, use Cmd+C / Ctrl+C and right-click → Copy; verify by pasting elsewhere.
5. Preferences: with terminal.enablePaste: false (resp. enableCopy: false), the corresponding keystroke and menu entry should no-op in the terminal but still work in editors. Reset and re-verify.
6. Non-terminal contexts: confirm Cmd+V / Cmd+C are unchanged in editors, Command Palette, dialogs, input fields.
7. fix(terminal): copy via context menu does not work #17290 regression check: right-click → Copy with a selection still copies.

Follow-ups

• Electron Edit menu bypasses Theia commands. electron-main-menu-factory.ts:240 deletes menuItem.execute when assigning role: 'paste' / 'copy', so Edit → Paste / Copy runs Electron's native role and skips the new terminal commands (and the terminal.enable* preferences). Paste still lands in xterm's textarea via the synthetic DOM event and reaches the PTY, so it works — but it isn't gated by the preferences. VSCode keeps a click handler alongside the role and re-dispatches through the renderer's keybinding service so a focused terminal resolves the correct command; aligning Theia's Electron menu factory with that pattern is tracked as a follow-up.
• customKeyHandler Cmd+V / Cmd+C branches are now mostly dead with the renderer-level keybindings handling both. Worth removing or scoping to browser-only modes in a follow-up.

Breaking changes

• This PR introduces breaking changes and requires careful review.

Note: TerminalWidget gains a new abstract method paste(text: string). Downstream subclasses will need to implement it. Consistent with getSelection() / hasSelection() added in #17290.

Attribution

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[cdamus]

Very clean! Nice fix and it all works as advertised and (perhaps more importantly) as I expect as a user.

I think, though, that adding an abstract method to a class that is meant to be subclassed is a breaking API change, so that should be called out in the changelog.

The two nits are optional.

[cdamus]

Thanks for the changes! 🚀