Collapse multi-line copy previews and clarify Alt+M help

Keep the status flash single-line for short drag-selects, and make the
help row describe mouse capture as a toggle with capture as the default.

Author: TheGreatAxios

src/tui/keybindings.ts

@@ -25,7 +25,7 @@ export const SHELL_SHORTCUTS: readonly ShellShortcut[] = [
   { keys: "Ctrl+C", description: "interrupt the run, or clear the prompt when idle; press twice to exit" },
   { keys: "Ctrl+G", description: "cancel the most recently queued or steered message before it dispatches" },
   { keys: "Alt+C", description: "copy mode: pick a message, tool output, or diff; press again to close it" },
-  { keys: "Alt+M", description: "hand the mouse to the terminal for native drag-select; on by default so wheel scroll, click-to-expand, and drag-to-copy work" },
+  { keys: "Alt+M", description: "toggle DEC mouse capture (on by default: wheel scroll, click-to-expand, drag-to-copy); off restores native terminal drag-select" },
   { keys: "Alt+E", description: "expand or collapse every collapsible row (tool call, diff, skill, reasoning)" },
   { keys: "Alt+T", description: "show or hide the task list above the prompt" },
   { keys: "Alt+O", description: "observe a live subagent session; a system row says so when there is none" },

src/tui/selection-copy.test.ts

@@ -76,4 +76,17 @@ describe("copyFinishedSelection", () => {
     expect(h.flashes[0]).toContain("…")
     expect(h.flashes[0]!.length).toBeLessThan(long.length + 40)
   })
+
+  test("collapses multi-line selections in the flash preview", () => {
+    const h = host()
+    const ok = copyFinishedSelection(h, {
+      isDragging: false,
+      getSelectedText: () => "ok\ngo",
+    })
+    expect(ok).toBe(true)
+    expect(h.clipboard.writes).toEqual(["ok\ngo"])
+    expect(h.flashes[0]).toContain("ok go")
+    expect(h.flashes[0]).not.toContain("\n")
+  })
 })
+

src/tui/selection-copy.ts

@@ -35,8 +35,11 @@ export function copyFinishedSelection(
   if (text.length === 0) return false
 
   void host.clipboard.writeText(text)
+  // Notice row is one line; always collapse whitespace so multi-line
+  // drag-selects do not inject raw newlines into chrome.
+  const oneLine = text.replace(/\s+/g, " ").trim()
   const preview =
-    text.length > 48 ? `${text.slice(0, 45).replace(/\s+/g, " ")}…` : text
+    oneLine.length > 48 ? `${oneLine.slice(0, 45)}…` : oneLine
   host.flash(`Copied ${text.length} chars: ${preview}`)
   host.clearSelection()
   return true