Lesson learned the hard way. Attach the heatsink to the LA4225 chip before soldering the chip to the PC board. Otherwise, inserting the chip down as far as it will go and soldering will result in the heatsink not being able to be attached to the chip.
And thanks to Allan KN4UD. My 3.5 " Nextion now has its display extending to the screen edges.
VK3DAP / ZL2DAP