fixes #358 EventMouse button mapping

mouse.go

@@ -1,4 +1,4 @@
-// Copyright 2015 The TCell Authors
+// Copyright 2020 The TCell Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use file except in compliance with the License.
@@ -79,11 +79,13 @@ func NewEventMouse(x, y int, btn ButtonMask, mod ModMask) *EventMouse {
 // emulations (such as vt100) won't support mice at all, of course.
 type ButtonMask int16
 
-// These are the actual button values.
+// These are the actual button values.  Note that tcell version 1.x reversed buttons
+// two and three on *nix based terminals.  We use button 1 as the primary, and
+// button 2 as the secondary, and button 3 (which is often missing) as the middle.
 const (
-	Button1 ButtonMask = 1 << iota // Usually left mouse button.
-	Button2                        // Usually the middle mouse button.
-	Button3                        // Usually the right mouse button.
+	Button1 ButtonMask = 1 << iota // Usually the left (primary) mouse button.
+	Button2                        // Usually the right (secondary) mouse button.
+	Button3                        // Usually the middle mouse button.
 	Button4                        // Often a side button (thumb/next).
 	Button5                        // Often a side button (thumb/prev).
 	Button6
@@ -94,4 +96,8 @@ const (
 	WheelLeft                 // Wheel motion to left.
 	WheelRight                // Wheel motion to right.
 	ButtonNone ButtonMask = 0 // No button or wheel events.
+
+	ButtonPrimary   = Button1
+	ButtonSecondary = Button2
+	ButtonMiddle    = Button3
 )

tscreen.go

@@ -919,10 +919,10 @@ func (t *tScreen) buildMouseEvent(x, y, btn int) *EventMouse {
 		button = Button1
 		t.wasbtn = true
 	case 1:
-		button = Button2
+		button = Button3 // Note we prefer to treat right as button 2
 		t.wasbtn = true
 	case 2:
-		button = Button3
+		button = Button2 // And the middle button as button 3
 		t.wasbtn = true
 	case 3:
 		button = ButtonNone