terminal: fix unreliable middle-click paste on Linux

On Linux the middle-click paste in the terminal was unreliable when the
click occurred over a word because xterm.js intercepts the mouse event
to select the word before the browser can perform the primary-selection
paste. Instead of relying on Chromium's primary-selection paste, this
change explicitly pastes the primary selection clipboard when
middleClickBehavior is 'default' on Linux, matching the behavior users
expect and making the paste reliable regardless of cursor position.

Fixes microsoft#272927

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: yogeshwaran-c

src/vs/workbench/contrib/terminalContrib/clipboard/browser/terminal.clipboard.contribution.ts

@@ -120,6 +120,15 @@ export class TerminalClipboardContribution extends Disposable implements ITermin
 					this.paste();
 					return { handled: true };
 				}
+				// On Linux with 'default' middle click behavior, explicitly paste the
+				// primary selection clipboard. Relying on the browser to handle the
+				// middle-click paste is unreliable when the click occurs over a word
+				// because xterm.js intercepts the mouse event to select the word before
+				// the browser can perform the paste.
+				if (isLinux && this._terminalConfigurationService.config.middleClickBehavior === 'default') {
+					this.pasteSelection();
+					return { handled: true };
+				}
 				break;
 			}
 			case 2: { // Right click