fix: avoid using initial step in validation

setting the min or max property should not trigger step-based validation

fix #420

src/vaadin-number-field.html

@@ -303,7 +303,13 @@
         }
 
         _stepChanged(step) {
-          this.inputElement.step = step;
+          if (!this.__stepChangedCalled) {
+            // Setting the initial value, avoid using it in validation.
+            this.inputElement.step = 'any';
+            this.__stepChangedCalled = true;
+          } else {
+            this.inputElement.step = step;
+          }
         }
 
         _minChanged(min) {

test/number-field.html

@@ -749,6 +749,49 @@
           expect(numberField.validate(), 'value should not be greater than max').to.be.false;
         });
 
+        it('should validate by step when defined by user', () => {
+          numberField.step = 1.5;
+
+          numberField.value = '0';
+          expect(numberField.validate()).to.be.true;
+
+          numberField.value = '-1.5';
+          expect(numberField.validate()).to.be.true;
+
+          numberField.value = '3';
+          expect(numberField.validate()).to.be.true;
+
+          numberField.value = '-1';
+          expect(numberField.validate()).to.be.false;
+
+          numberField.value = '2';
+          expect(numberField.validate()).to.be.false;
+        });
+
+        it('should use min as step basis in validation when both are defined', () => {
+          numberField.min = 1;
+          numberField.step = 1.5;
+
+          numberField.value = '1';
+          expect(numberField.validate()).to.be.true;
+
+          numberField.value = '2.5';
+          expect(numberField.validate()).to.be.true;
+
+          numberField.value = '4';
+          expect(numberField.validate()).to.be.true;
+
+          numberField.value = '1.5';
+          expect(numberField.validate()).to.be.false;
+        });
+
+        it('should not validate by step when only min and max are set', () => {
+          numberField.min = 1;
+          numberField.max = 5;
+          numberField.value = 1.5; // would be invalid by default step=1
+          expect(numberField.validate()).to.be.true;
+        });
+
         describe('removing validation constraints', () => {
           it('should update "invalid" state when "min" is removed', () => {
             numberField.value = '42';