open window keeps automatically rezising when i click off it

Example: If I am using one window and it's taking up half my screen or so, and then I click another separate window, the first one will automatically resize to fit the whole screen.

It's driving me demented. Please, can anyone help? It's really ruining my user experience, and I'm so frustrated with it because I'm spending ages each time I use my laptop resizing the windows after they've moved autonomously.