fix: when window resize trigger resizeEventHandler, placeholder shouldn't hide (#46)

[CodeMd2z]

When window resize event trigger the handler, it will try to close the placeholder by set isDragging to false, which should only closed by resizeend event, then cause placeholder closed unexpected

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[CodeMd2z]

to fix #46

[CodeMd2z]

@qmhc Hope check this soon.

[qmhc]

你需要把你 fork 相关的描述移除掉我才能合入这个 PR

[CodeMd2z]

fixed to new PR #49