Table: use Range to determine text overflow (#11124)

packages/table/src/table-body.js

 import { getCell, getColumnByCell, getRowIdentity } from './util';
-import { hasClass, addClass, removeClass } from 'element-ui/src/utils/dom';
+import { getStyle, hasClass, addClass, removeClass } from 'element-ui/src/utils/dom';
 import ElCheckbox from 'element-ui/packages/checkbox';
 import ElTooltip from 'element-ui/packages/tooltip';
 import debounce from 'throttle-debounce/debounce';
@@ -336,8 +336,15 @@ export default {
 
       // 判断是否text-overflow, 如果是就显示tooltip
       const cellChild = event.target.querySelector('.cell');
-
-      if (hasClass(cellChild, 'el-tooltip') && cellChild.scrollWidth > cellChild.offsetWidth && this.$refs.tooltip) {
+      // use range width instead of scrollWidth to determine whether the text is overflowing
+      // to address a potential FireFox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1074543#c3
+      const range = document.createRange();
+      range.setStart(cellChild, 0);
+      range.setEnd(cellChild, 1);
+      const rangeWidth = range.getBoundingClientRect().width;
+      const padding = (parseInt(getStyle(cellChild, 'paddingLeft'), 10) || 0) +
+        (parseInt(getStyle(cellChild, 'paddingRight'), 10) || 0);
+      if (hasClass(cellChild, 'el-tooltip') && rangeWidth + padding > cellChild.offsetWidth && this.$refs.tooltip) {
         const tooltip = this.$refs.tooltip;
         // TODO 会引起整个 Table 的重新渲染，需要优化
         this.tooltipContent = cell.textContent || cell.innerText;