Fix handling of empty selection in the 2D plot snip

2D plot snips allow the user to select a region and zoom the plot to that
region -- Sometimes, when the user just clicks on the snip, the system
interprets this as a selection and the snip attempts to zoom to an empty
region.  Empty regions have +nan.0 values, and the code fails with an
exception when it attempts to covert the +nan.0 to an exact number.

plot-gui-lib/plot/private/gui/snip2d.rkt

@@ -107,8 +107,14 @@
     (define (zoom-or-unzoom)
       (cond [dragging?
              (set! dragging? #f)
-             (define new-rect (area-bounds->plot-bounds (get-new-area-bounds-rect)))
-             (cond [(and (rect-rational? new-rect) (not (rect-zero-area? new-rect)))
+             (define new-rect
+               (let ([bounds (get-new-area-bounds-rect)])
+                 ;; BOUNDS might be +nan.0 for an empty selection
+                 (and (rect-rational? bounds)
+                      (area-bounds->plot-bounds bounds))))
+             (cond [(and new-rect
+                         (rect-rational? new-rect)
+                         (not (rect-zero-area? new-rect)))
                     #;(printf "~a: new-plot-bounds-rect = ~v~n"
                               (current-milliseconds) new-rect)
                     (set! plot-bounds-rects (cons plot-bounds-rect plot-bounds-rects))

plot-test/plot/tests/PRs/snip-empty-selection.rkt

@@ -0,0 +1,56 @@
+#lang racket
+(require plot racket/gui/base racket/draw rackunit)
+
+;; 2D interactive plots would occasionally thrown an exception if the user
+;; clicked on the plot.  This was because the system would interpret it as a
+;; drag-select operation which started and finished at the same place,
+;; resulting in an empty selection -- empty selections are represented using
+;; +nan.0, which cannot be converted to an exact number.
+;;
+;; In this test we simulate an empty selection using mouse events sent
+;; directly to the snips `on-event` method.
+
+(define empty-selection-zoom
+  (test-suite
+   "empty-selection-zoom"
+   (test-case "empty-selection-zoom"
+
+     ;; Scaffolding, construct a pasteboard to hold our plot snip
+     (define tl (new frame% [label "hello"] [width 800] [height 600]))
+     (define pb [new pasteboard%])
+     (define editor (new editor-canvas% [parent tl] [editor pb]))
+
+     ;; Construct the plot snip and add it to the pasteboard so it has an
+     ;; administrator.
+     (define snip (plot-snip (function sin -3 3)))
+     (send pb insert snip)
+
+     ;; Show the frame -- this is not strictly needed, but will ensure that
+     ;; all widgets have their proper dimensions set and mouse events will be
+     ;; "interpreted" according to correct snip positions.
+     (send tl show #t)
+
+     ;; Construct a dummy DC and the left-click, drag, left-release events
+     ;; which will simulate the drag-selection on the snip.  Note that the
+     ;; start and end events have the same X, Y coordinates, which will result
+     ;; in an empty rectangle being selected.
+     (define dc (new record-dc% [width 800] [height 600]))
+     (define click (new mouse-event% [event-type 'left-down] [x 10] [y 10]))
+     (define drag (new mouse-event% [event-type 'motion] [left-down #t] [x 11] [y 11]))
+     (define unclick (new mouse-event% [event-type 'left-up] [x 10] [y 10]))
+
+     ;; Send the snip the events -- the snip will think the user is selecting
+     ;; a region on the plot.
+     (send snip on-event dc 0 0 0 0 click)
+     (send snip on-event dc 0 0 0 0 drag)
+     (after
+      (check-not-exn
+       ;; zoom is triggered when the user releases the mouse event -- this
+       ;; used to throw an exception as it tried to operate on an empty
+       ;; rectangle, containing +nan.0 numbers.
+       (lambda () (send snip on-event dc 0 0 0 0 unclick)))
+      (send tl show #f)))))
+
+(module+ test
+  (require rackunit/text-ui)
+  (run-tests empty-selection-zoom))