When viewing in windowed mode, I desired to snap my Minecraft window to one half of the screen so I could look something up while still keeping a close eye on what went on while I was away. However, when I went to click on my Minecraft window after searching for what needed to be searched, the dimensions snapped back to default window dimensions. To replicate, simply open up Minecraft in windowed mode, change its screen dimensions, click on something else, then click on the Minecraft window again.