EDIT: DONE! This is a working engine. Huge thanks to Magistross retrodude and newt
* Always integer pixel positions
* Always fall down when trying to cross a gap of equal width to the player, at any horizontal speed
* Jumpthrough platforms
* loop-based movement
* framerate independent (takes dt into account)
* controllable speed (no acceleration, on/off)
* collision detection with walls
So I revisited this ( https://www.scirra.com/forum/viewtopic.php?f=147&t=81757). It's a very old-school platform engine suitable for pixel games, that moves the player one pixel at a time using loops, always integer positions (unlike the built-in platform behavior). The player can fit inside gaps that are the same size as itself without horizontal or vertical wiggle room.
One thing I haven't been able to fix is falling through gaps when moving. If player.horizontalSpeed is high enough, player never registers the gap under its feet.
I could try to solve this by using R0J0hound 's solution https://www.scirra.com/forum/how-do-i-fall-between-blocks_t125214?&hilit=platform+gaps, since horizontal walls work flawlessly with this engine.
However I wonder if it can be solved another way, since there's already a loop in place. Anyone?
Here is the CAPX: