feat(clipboard): add middle-click paste from X11 primary selection

Adds readPrimary() function supporting wl-paste, xclip, and xsel
for reading the X11 primary selection buffer. Adds middle-click
handler to the prompt textarea.

Upstream: anomalyco/opencode#16379

https://claude.ai/code/session_01R2NB6YZNAFe57T3uhSCGo9

Author: claude

packages/core/src/components/prompt/index.tsx

@@ -11,6 +11,7 @@
  */
 
 import { readFile, stat } from "node:fs/promises";
+import { platform } from "node:os";
 import { basename } from "node:path";
 
 import { useCallback, useEffect, useMemo, useRef, useState } from "react";
@@ -565,6 +566,25 @@ export function Prompt({
     commandContext,
   ]);
 
+  const handleMouseDown = useCallback(
+    async (e: { button?: number; preventDefault?: () => void }) => {
+      if (e.button !== 1) {
+        return; // 1 = middle button
+      }
+      if (disabled) {
+        return;
+      }
+      e.preventDefault?.();
+      const text = await Clipboard.readPrimary();
+      const textarea = textareaRef.current;
+      if (!(text && textarea)) {
+        return;
+      }
+      textarea.insertText(text);
+    },
+    [disabled]
+  );
+
   const handleKeyDown = useCallback(
     // biome-ignore lint/complexity/noExcessiveCognitiveComplexity: Complex keyboard event handling
     async (e: KeyEvent) => {
@@ -967,6 +987,7 @@ export function Prompt({
               minHeight={1}
               onContentChange={handleContentChange}
               onKeyDown={handleKeyDown}
+              onMouseDown={handleMouseDown}
               onPaste={handlePaste}
               onSubmit={handleSubmit}
               placeholder={displayPlaceholder}

packages/core/src/lib/clipboard.ts

@@ -208,6 +208,39 @@ const getCopyMethod = lazy(() => {
   };
 });
 
+async function readPrimary(): Promise<string | undefined> {
+  const os = platform();
+  if (os === "linux") {
+    if (
+      process.env.WAYLAND_DISPLAY &&
+      which.sync("wl-paste", { nothrow: true })
+    ) {
+      const result = await execa("wl-paste", ["--primary", "--no-newline"], {
+        reject: false,
+      });
+      if (result.stdout) {
+        return result.stdout;
+      }
+    }
+    if (which.sync("xclip", { nothrow: true })) {
+      const result = await execa("xclip", ["-selection", "primary", "-o"], {
+        reject: false,
+      });
+      if (result.stdout) {
+        return result.stdout;
+      }
+    }
+    if (which.sync("xsel", { nothrow: true })) {
+      const result = await execa("xsel", ["--primary", "--output"], {
+        reject: false,
+      });
+      if (result.stdout) {
+        return result.stdout;
+      }
+    }
+  }
+}
+
 async function copy(text: string): Promise<void> {
   writeOsc52(text);
   await getCopyMethod()(text);
@@ -216,4 +249,5 @@ async function copy(text: string): Promise<void> {
 export const Clipboard = {
   read,
   copy,
+  readPrimary,
 };