Fix: right-click menu problem on windows, focus change parent scrollTop

[z-ZYS-s]

1. It costs more time to show right-click menu in browsers on windows.
2. There are some problems when switching copy and paste menus.

Following changes are for these two bugs.

that.element.contentEditable = true;
setTimeout(function() {
    that.element.contentEditable = 'inherit'; // 'false';
    var selection = utils.getSelection();
    // If user doesn't select text
    if (!selection || (Terminal.isMac && selection === ' '))
        that.inputElement.focus();
}, Terminal.isMac ? 1 : 400);

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1. There is a bug when window is smaller than a terminal and some parent node of this terminal like can be scrolled. If you scroll in the bottom and then click the terminal, it will jump to the top. This is because .terminal-input textarea focused is on the top.

-    position: absolute;
-    top: 0;
+    position: fixed;

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All bugs have been tested in our product.