Testing this with floating windows, it seems I can't click on anything outside a rectangle in the top-left corner slightly larger than a quarter of the screen (edit: panels are clickable). Though the size of the clickable area seems to be variable (I think it changes when maximizing and unmaximizing an app?).
Tiling and maximized windows seem to behave as expected.
Clicking on Workspaces in Settings in the image below will focus the Firefox window behind (also I couldn't click the attach button in GitHub, regardless of window position, even though it was highlighted on hover).

Edit: But I can confirm that this also fixes #418. 🙃

Thanks for testing! Last commit should fix the bugginess around floating windows.

The unclickable area issue seems to be fixed! Though there still seem to be some focus issues, where clicking on a window in the background doesn't focus it and bring it to the front (though they are interactable), and input sometimes goes through to the window below.

Clicking on the cosmic files or Nautilus window here doesn't focus them and bring them to the front.
While trying to attach this screenshot, clicking on the Open button in the cosmic-files dialog just focused (and brought to front) the (maximized) Firefox window below, until the dialog was moved towards the top-left screen corner.

Edit: Input seems to only go through when there's a maximized window below, and it goes through only outside the similarly sized top-left anchored rectangle as above. With tiling, clicks in a floating window in front sometimes visually move focus to the window behind.

Yeah, stupid of me, the element_under code for floating windows (related to keyboard focus) needed the exact same fixes as surface_under (related to pointer focus) before.

I'm not encountering any issues from some initial testing with the latest changes. :)
Edit: Testing with a game, it seems the cursor is offset from it's actual position, and can't be moved all the way to the top-left corner (this doesn't happen with current master). Though it might be game-specific (I only have one installed for testing on COSMIC).
But this could also be an interaction between the weird window behavior on game startup and this PR (and so the fix might be out-of-scope for this PR).

I guess render_input_order should just return an iterator, but that's maybe awkward until rust has a generator syntax with yield. Seems like a good way to do things for now.

@git-f0x Good observation, the new code failed to take fullscreen surfaces correctly into account.

Seems to work fine after a day of extensive testing / running in production. Lets merge