Minimising Glitch when I use Search Key in Long Documents

This is my problem:

I have long text  documents on my computer. When I open a document  (in order to search it) and then click the Search Icon, the document minimises and flashes on the taskbar.   I have to click the minimised icon on the taskbar to restore the document. And sometimes the Search box also minimises onto the taskbar. 

 

On the other hand if I open the same document and then click on the text, or briefly scroll down text, the document acts as I would expect it to and there is NO problem when I use the Search key.