Support Shift+Click to extend the current terminal text selection (iTerm2-style)

[cola-runner]

Problem

Warp's terminal can't extend an existing text selection with Shift+Click the way iTerm2 (and most terminals) can. Today every left-click in the block list starts a brand-new selection, so there's no way to:

• click a start point, scroll far down a long command's output, then Shift+Click an end point to select everything in between, or
• nudge the end of an existing selection outward without re-dragging the whole thing from scratch.

For long / multi-screen output this makes selecting a large range painful — you have to drag and rely on auto-scroll the entire way, and overshooting means starting over.

Steps to reproduce

1. Run a command that produces a few screenfuls of output.
2. Click (or drag-select) somewhere near the top of the output.
3. Scroll down, hold Shift, and left-click near the bottom.

Expected: the selection extends from the original anchor to the clicked point (iTerm2 behavior).
Actual: the previous selection is discarded and a new (empty) selection starts at the click.

Expected behavior

With a selection already present, Shift + left-click should extend the selection to the clicked point, keeping the original anchor fixed (and you can then drag to fine-tune). A plain click still starts a new selection.

Notes

The selection model already has the needed primitive: dragging uses an "update the tail, keep the head" operation, while Shift+Click currently falls through to "start a new selection" instead of being wired to it.

I have a fix for this (gated behind a feature flag, scoped to the block list / scrollback for v1) and will open a PR linked to this issue.

[oz-for-oss]

@cola-runner

This issue appears to overlap with existing issues:

• Support Shift+Click to extend existing text selection in terminal output #9963 — Support Shift+Click to extend existing text selection in terminal output
• shift+click to select text #4715 — shift+click to select text
• Shift+click for text selection doesn't work #3426 — Shift+click for text selection doesn't work

If this report is meaningfully different, please comment with the additional context or distinguishing behavior so a maintainer can review it. Otherwise, a maintainer may close it as a duplicate after review.

Maintainer details

I concluded that feature request for iTerm2-style Shift+Click terminal text-selection extension; this duplicates existing open requests for the same behavior..

Duplicate reasoning

• Support Shift+Click to extend existing text selection in terminal output #9963: Requests the same terminal-output workflow where holding Shift and clicking should extend an existing selection from its original anchor to the clicked location.
• shift+click to select text #4715: Describes selecting existing terminal text and using Shift+Click to automatically select through the clicked point, matching this issue's expected behavior.
• Shift+click for text selection doesn't work #3426: Reports that Shift+Click should select text between the original position and a later click in previous terminal blocks, which is the same scrollback/block-list selection request.

This is my automated analysis and may be incorrect. A maintainer will verify the details.

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[cola-runner]

@cla-bot check

[leesa-ui-w]

Hey @cola-runner, thanks for the write-up and PR #11933! Same as #3426, closing this one to consolidate. Your PR is still visible.