Tab: fix tabs-item's padding cause active-bar's width and transform err( … #15355) (#16520)

* Tab: Refactor tabs-item's padding with dynamic value

packages/tabs/src/tab-bar.vue

@@ -32,16 +32,17 @@
               return true;
             } else {
               tabSize = $el[`client${firstUpperCase(sizeName)}`];
+              const tabStyles = window.getComputedStyle($el);
               if (sizeName === 'width' && this.tabs.length > 1) {
-                tabSize -= (index === 0 || index === this.tabs.length - 1) ? 20 : 40;
+                tabSize -= parseFloat(tabStyles.paddingLeft) + parseFloat(tabStyles.paddingRight);
+              }
+              if (sizeName === 'width') {
+                offset += parseFloat(tabStyles.paddingLeft);
               }
               return false;
             }
           });
 
-          if (sizeName === 'width' && offset !== 0) {
-            offset += 20;
-          }
           const transform = `translate${firstUpperCase(sizeDir)}(${offset}px)`;
           style[sizeName] = tabSize + 'px';
           style.transform = transform;