Don't send mouse events when selection is forced

Fixes #1090

src/SelectionManager.ts

@@ -307,6 +307,15 @@ export class SelectionManager extends EventEmitter implements ISelectionManager
     return (offset / Math.abs(offset)) + Math.round(offset * (DRAG_SCROLL_MAX_SPEED - 1));
   }
 
+  /**
+   * Returns whether the selection manager should force selection, regardless of
+   * whether the terminal is in mouse events mode.
+   * @param event The mouse event.
+   */
+  public shouldForceSelection(event: MouseEvent): boolean {
+    return Browser.isMac ? event.altKey : event.shiftKey;
+  }
+
   /**
    * Handles te mousedown event, setting up for a new selection.
    * @param event The mousedown event.
@@ -325,9 +334,7 @@ export class SelectionManager extends EventEmitter implements ISelectionManager
 
     // Allow selection when using a specific modifier key, even when disabled
     if (!this._enabled) {
-      const shouldForceSelection = Browser.isMac ? event.altKey : event.shiftKey;
-
-      if (!shouldForceSelection) {
+      if (!this.shouldForceSelection(event)) {
         return;
       }