How to stop apps collapsing while typing?

Hi I recently bought samsung j3. I love the phone, however have an issue...whenever I am typing...in any app could be WhatsApp, Google search, even typing this...the page collapses of it's own accord and brings me to multiple tabs. This is so frustrating and I have lost long bodies of text due to this! I don't know if its something I'm pressing accidentally or some other issue but it is driving me crazy! Please help thanks. *the apps are not crashing

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