Adjust the mouse drag threshold
When you click something, it's not uncommon for your hand to move a little between the time you press the mouse button and the time you release it. For this reason, dragging only starts if you move the pointer past a certain threshold, so that you don't accidentally start dragging every time you click. You can control the minimum distance required to start dragging.
Click the icon at the very right of the menu bar and select System Settings.
Click Mouse and Touchpad.
Under Drag and Drop, adjust the Threshold slider to a value you find comfortable. Try moving the settings window by dragging the titlebar to test the current value.
This setting will affect both your mouse and touchpad, as well as any other pointing device.