util: let wlr_box_closest_point() be within 1/65536 of right/bottom edge

Limiting the position to (x + width - 1, y + height - 1) created a 1px
"dead zone" at monitor edges, noticeable with high-resolution mice with
motion deltas of <1px.

See: https://github.com/swaywm/sway/issues/8110

Using (x + width - 1/65536, y + height - 1/65536) instead should make
the "dead zone" small enough to be unobservable, while the value 1/65536
is still large enough to avoid rounding to zero (due to loss of
significant digits) in simple floating-point calculations.

This does expose a client-side bug in Qt layer-shell applications,
noticeable in right/bottom panels which do not accept positions beyond
(x + width - 1, x + height - 1) as valid - thus driving the cursor
to the bottom/right of the screen to click on the panel does not work.
I don't have a good workaround for this, and probably it needs to be
fixed in Qt itself.

Fixes: 3fc66d4525
("util: fix non-linear behavior of wlr_box_closest_point()")

util/box.c

@@ -14,29 +14,21 @@ void wlr_box_closest_point(const struct wlr_box *box, double x, double y,
 	}
 
 	// Note: the width and height of the box are exclusive; that is,
-	// for a 100x100 box at (0,0), the point (99,99) is inside it
+	// for a 100x100 box at (0,0), the point (99.9,99.9) is inside it
 	// while the point (100,100) is outside it.
 	//
-	// Mathematically, there exists no single closest point to the
+	// In order to be consistent with e.g. wlr_box_contains_point(),
-	// bottom-right corner of the box while remaining inside it. You
+	// this function returns a point inside the bottom and right edges
-	// can construct an infinite series approaching the limit, such
+	// of the box by at least 1/65536 of a unit (pixel). 1/65536 is
-	// as {(99,99), (99.9,99.9), (99.99,99.99)...}, but since the
+	// small enough to avoid a "dead zone" with high-resolution mice
-	// intervals are half-open, there is no "last" point.
+	// but large enough to avoid rounding to zero (due to loss of
-	//
+	// significant digits) in simple floating-point calculations.
-	// This function must therefore define an arbitrary "closest"
-	// point. For simplicity and consistency, this is defined to be
-	// (box.x + width - 1, box.y + height - 1).
-	//
-	// (The previous implementation was non-linear: with the example
-	// 100x100 box, it would return an input point of (99.9,99.9)
-	// unchanged, but for an input point (100.1,100.1) the returned
-	// point would jump back to (99.0,99.0). This is now fixed.)
 
 	// find the closest x point
 	if (x < box->x) {
 		*dest_x = box->x;
-	} else if (x > box->x + box->width - 1) {
+	} else if (x > box->x + box->width - 1/65536.0) {
-		*dest_x = box->x + box->width - 1;
+		*dest_x = box->x + box->width - 1/65536.0;
 	} else {
 		*dest_x = x;
 	}
@@ -44,8 +36,8 @@ void wlr_box_closest_point(const struct wlr_box *box, double x, double y,
 	// find closest y point
 	if (y < box->y) {
 		*dest_y = box->y;
-	} else if (y > box->y + box->height - 1) {
+	} else if (y > box->y + box->height - 1/65536.0) {
-		*dest_y = box->y + box->height - 1;
+		*dest_y = box->y + box->height - 1/65536.0;
 	} else {
 		*dest_y = y;
 	}