Don't position popups at negative x co-ordinates

basis/math/rectangles/positioning/positioning-tests.factor

@@ -36,4 +36,11 @@ IN: math.rectangles.positioning.tests
     { 50 50 }
     { 100 100 }
     popup-loc
+] unit-test
+
+[ { 0 20 } ] [
+    { -50 70 } { 0 0 } <rect>
+    { 50 50 }
+    { 100 100 }
+    popup-loc
 ] unit-test

basis/math/rectangles/positioning/positioning.factor

@@ -7,7 +7,7 @@ IN: math.rectangles.positioning
 ! in a semi-intelligent manner
 
 : popup-x ( visible-rect popup-dim screen-dim -- x )
-    [ loc>> first ] 2dip swap [ first ] bi@ - min ;
+    [ loc>> first ] 2dip swap [ first ] bi@ - min 0 max ;
 
 : preferred-y ( visible-rect -- y )
     [ loc>> ] [ dim>> ] bi [ second ] bi@ + ;