fix icon visibility for vertical tabs

.gitignore

@@ -27,6 +27,7 @@ yarn-error.log
 docs/api
 .travis.ssh.key
 *.code-workspace
+.idea/
 
 .electron-symbols
 sentry.properties

tabby-core/src/components/tabHeader.component.scss

@@ -3,7 +3,7 @@ $tabs-height: 38px;
 :host {
     position: relative;
     cursor: pointer;
-    box-sizing: content-box;
+    box-sizing: border-box;
 
     > * { cursor: pointer; }