Ensure linkifier cancels mouse event when being handled

The terminal mouse events handling code was being run before links were
being considered. This caused clicking link to have unexpected behavior
in mouse mode term apps.

Fixes #1020

src/input/MouseZoneManager.ts

@@ -21,7 +21,6 @@ export class MouseZoneManager implements IMouseZoneManager {
 
   private _areZonesActive: boolean = false;
   private _mouseMoveListener: (e: MouseEvent) => any;
-  private _mouseDownListener: (e: MouseEvent) => any;
   private _clickListener: (e: MouseEvent) => any;
 
   private _tooltipTimeout: number = null;
@@ -31,6 +30,8 @@ export class MouseZoneManager implements IMouseZoneManager {
   constructor(
     private _terminal: ITerminal
   ) {
+    this._terminal.element.addEventListener('mousedown', e => this._onMouseDown(e));
+
     // These events are expensive, only listen to it when mouse zones are active
     this._mouseMoveListener = e => this._onMouseMove(e);
     this._clickListener = e => this._onClick(e);
@@ -140,11 +141,30 @@ export class MouseZoneManager implements IMouseZoneManager {
     }
   }
 
+  private _onMouseDown(e: MouseEvent): void {
+    // Ignore the event if there are no zones active
+    if (!this._areZonesActive) {
+      return;
+    }
+
+    // Find the active zone, prevent event propagation if found to prevent other
+    // components from handling the mouse event.
+    const zone = this._findZoneEventAt(e);
+    if (zone) {
+      // TODO: When link modifier support is added, the event should only be
+      // cancelled when the modifier is held (see #1021)
+      e.preventDefault();
+      e.stopImmediatePropagation();
+    }
+  }
+
   private _onClick(e: MouseEvent): void {
+    // Find the active zone and click it if found
     const zone = this._findZoneEventAt(e);
     if (zone) {
       zone.clickCallback(e);
       e.preventDefault();
+      e.stopImmediatePropagation();
     }
   }