There's a few possibilities here. The first is the nylon pad on the mid frame has come off (green outline). Make sure its present and the bottom cover is not caved in.
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As you commented on pressing on the uppercase likewise causes issues, you may have a cold solder joint from the PCH chip's LVDS connection point as well as the LVDS connector and LVDS display cable.
The 13" models use the onboard graphics services within the CPU (Intel HD Graphics 4000)