OSC 52: Gate clipboard writes on focus to prevent background clipboard hijacking

[DivInstance]

This change scopes OSC 52 clipboard writes to when the terminal has focus.

Changes:

1. src/cascadia/TerminalCore/Terminal.hpp: add _isFocused flag.
2. src/cascadia/TerminalCore/Terminal.cpp: update FocusChanged to track focus state.
3. src/cascadia/TerminalCore/TerminalApi.cpp: require _isFocused alongside _clipboardOperationsAllowed in CopyToClipboard.

Rationale:

1. Addresses request in #19051 (“OSC 52 should only function while the terminal has focus”).
2. Prevents background sessions from silently writing to the system clipboard.
3. Notes:

Behavior remains unchanged when the window is focused.

1. This only gates VT-driven clipboard writes (OSC 52), not user-initiated copy.

Testing:

1. Verified clipboard write via OSC 52 succeeds when focused, does nothing when unfocused. Lint passed locally.

Link: Closes #19051.

[DivInstance]

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[j4james]

If we think this is an important security feature, should it not also be applied to conhost's OSC 52 implementation? The equivalent method is here:

https://github.com/microsoft/terminal/blob/e80aadd98ba2a461cc5063dac1895b5f535dcff2/src/host/outputStream.cpp#L283

If you want to leave that for a followup, it may be best to keep the issue open so we don't forget about it.

ETA: You could possibly even cover both at the same time by having the check in AdaptDispatch::SetClipboard, and saving the focus state in the TerminalInput class. Although I'm not sure the TerminalInput::HandleFocus is guaranteed to be called in conhost, so it may require a bit of refactoring to make that work.

[DHowett]

@DivInstance Your pull request was approved and we were having a discussion about how to handle the same behavior elsewhere. Why'd you close it? :)

[DivInstance]

Thanks a lot for the feedback! I actually closed the PR by mistake while I was still figuring out the GitHub workflow sorry about that.

On your suggestions:

1. @lhecker Optimization for redundant calls – Great catch! I’ll add the if (_isFocused != focused) check so we can avoid those extra HandleFocus calls.

2. @j4james Conhost implementation – I completely agree this should apply to conhost as well. Would you prefer I:

• Add the same focus check to ConhostInternalGetSet::CopyToClipboard in src/host/outputStream.cpp, or

• Go with the refactored approach and put the check in AdaptDispatch::SetClipboard?

3. @DHowett, I’ll make sure #19051 is open until both parts are addressed.

Would you like me to take care of both changes in this PR, or should I split conhost into a follow-up? Also, I really appreciate your guidance here this is my first contribution, so the mentorship means a lot!

[DivInstance]

@j4james I’ve updated ConhostInternalGetSet::CopyToClipboard in (commit 513606d) to only allow OSC 52 clipboard writes when the console has focus, using the existing CONSOLE_HAS_FOCUS flag.

Appreciate it if you could review and share feedback on this whenever convenient.