add ScrollingBoxen unit test

tests/scrolling.cpp

@@ -111,8 +111,23 @@ TEST_CASE("Scrolling") {
     CHECK(0 == notcurses_render(nc_));
   }
 
-  // FIXME add one verifying boxes don't exceed the right side
-  // FIXME add one where we go past the end of the plane
+  // ensure that if we draw a box on a scrolling plane, it stops at the right
+  // side, as opposed to scrolling and making a horrible mess
+  SUBCASE("ScrollingBoxen") {
+    struct ncplane* n = ncplane_new(nc_, 4, 20, 1, 1, nullptr);
+    REQUIRE(n);
+    // verify that the new plane was started without scrolling
+    CHECK(!ncplane_set_scrolling(n, true));
+    cell ul = CELL_TRIVIAL_INITIALIZER, ur = CELL_TRIVIAL_INITIALIZER;
+    cell dl = CELL_TRIVIAL_INITIALIZER, dr = CELL_TRIVIAL_INITIALIZER;
+    cell hl = CELL_TRIVIAL_INITIALIZER, vl = CELL_TRIVIAL_INITIALIZER;
+    CHECK(0 == cells_double_box(n, 0, 0, &ul, &ur, &dl, &dr, &hl, &vl));
+    CHECK(0 > ncplane_box_sized(n, &ul, &ur, &dl, &dr, &hl, &vl, 2, 25, 0));
+    CHECK(0 > ncplane_box_sized(n, &ul, &ur, &dl, &dr, &hl, &vl, 2, 21, 0));
+    CHECK(0 == notcurses_render(nc_));
+  }
+
+  // FIXME add one where we go past the end of the plane and force a new line
 
   CHECK(0 == notcurses_stop(nc_));
   CHECK(0 == fclose(outfp_));