Fix scroll bar moving in AnimationPlayer editor

Fix #7196

(cherry picked from commit babc5048dd23e80c9fb4a41a3f482ec8a8900ab9)

scene/gui/scroll_bar.cpp

@@ -832,7 +832,7 @@ ScrollBar::ScrollBar(Orientation p_orientation)
 
 	if (focus_by_default)
 		set_focus_mode( FOCUS_ALL );
-
+	set_step(0);
 
 }