I don't understand why we're all asking if it should be up to the compositor or app to set a window's position. The only correct answer is that it should be up to the user, so I don't see what's wrong with my suggestion of a "set window size request" function. Waylands idea of not letting apps set their window position because it thinks it knows better is very similar to what's wrong with commercial operating systems nowadays, and it's probably why many of us left Windows. For example, you could only disable "live security" in the settings for up to 10 minutes to speed up a download, and you aren't given an option to never update the operating system (if you wanted to increase the life expectancy of your SSD, for example).
The only compromise in this case that truly gives users full control is one where users get to configure their compositor to either. 1) Allow apps to freely set their positions 2) Have the window manager make them start in the center or at their last location 3) Use the position apps request to make smart decisions about how the app should be placed (for example, if an app always starts at (100, 100) on a 1000x700 monitor, but the user has recently switched to a new 3000x2400 monitor, then the app can be placed at (300, 300)) The user can also configure individual apps to start in different ways if they chose to allow apps to pick their positions, or some compositors can even give users the option to apply the above to certain apps separately. This design gives everyone 200% of what they asked for. Compositors can obtain even more information about how a window should be placed, and windows can choose their own locations knowing that the compositor will optimize that position for the users screen to make sure they don't make any mistakes. I don't see a single thing that this takes away from Waylands current design. Also, as Igor said, flags on how the compositor should interpret the size request would be a great feature as well. There could be flags for priority, relative/absolute positioning, or any number of other things that could be added in the future. (Qt uses a genius flag system, where each flag is double the last one so they can just be added to each other and no two combinations of them will match. For example: NO_FLAGS = 0x00, FLAG_A = 0x01, FLAG_B = 0x02, FLAG_C = 0x04, FLAG_D = 0x08, FLAG_E = 0x10, FLAG_F = 0x20, etc..., and then the flags could be used like "FLAG_B | FLAG_E" ("|" is an uncommon C/C++ feature for adding constants without optimization). It's probably irrelevant but it's really cool)