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util: fix non-linear behavior of wlr_box_closest_point()

John Lindgren requested to merge jlindgren90/wlroots:closest-point into master

Per comments in util/box.h, the width and height of a wlr_box are exclusive; that is, for a 100x100 box at (0,0), the point (99,99) is inside it while the point (100,100) is outside it.

Thus mathematically, there exists no single closest point to the bottom-right corner of the box while remaining inside it. You can construct an infinite series approaching the limit, such as {(99,99), (99.9,99.9), (99.99,99.99)...}, but since the intervals are half-open, there is no "last" point.

wlr_box_closest_point() must therefore define an arbitrary "closest" point. For points below and to the right of the box, the current implementation returns (box.x + width - 1, box.y + height - 1). Let's continue to do this.

However, the current implementation is non-linear: with the example 100x100 box, it will return an input point of (99.9,99.9) unchanged, but for an input point (100.1,100.1) the returned point will jump back to (99.0,99.0).

In practice, this non-linearity results in strange behaviors when driving the mouse cursor to a screen corner. On a 1920x1080 display for example, driving the cursor quickly to the bottom-left corner results in a position of exactly (0,1079). Continuing to slowly nudge the cursor downward results in the position jumping between (0,1079) and other, fractional coordinates such as (0,1079.88).

The fractional coordinates expose some client/toolkit-side bugs (which, to be clear, should be fixed on the client side), but IMHO the wlroots behavior is also inconsistent and wrong -- when I drive the mouse cursor into the corner of the screen, it should come to a stop at a fixed position, not jitter around.

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