Going by the unexpected touch behaviour you provide it seems that your multi-touch screen is not responding properly to your inputs for this you need to confirm it by doing a small and freely available multi touch test.
Download the app and check whether the touch is shown exactly where you selected. The two dots are drawn exactly where the phone reports you are touching.
or else you can simply do the following
1- Go to the developer options section in settings.
2- enable pointer location.
3- touch the screen as lightly as you can. you will notice a red number on the upper right corner of the screen named ""Prs:". this indicates the level of contact needed to register a touch.
4- do the same on different areas of the screen & remember try to touch the screen as lightly as you can (the goal here is to find the lower number that registers a touch)
If you see that at one point of the screen the screen is not responding according to your input then you need to get in contact with your Authorised service centre and report the issue.