Fix: Tab - Vertical tab with tab group #1095

Author: black7375

css/leptonChrome.css

@@ -12,6 +12,19 @@ $ucTabBackgroundShadowHard: drop-shadow(-1px 0px 0px var(--uc-tab-shadow-color,
   --uc-tab-shadow-color-bundle: var(--tab-line-color, var(--lwt-tab-line-color, rgba(128, 128, 142, 0.9)));
 }
 
+#tabbrowser-tabs:not([noshadowfortests]) {
+  .tab-background:is([selected], [multiselected]) {
+    box-shadow: unset !important;
+    filter: $ucTabBasicShadow;
+  }
+
+  /* filter create new stacking context */
+  &[expanded][orient="vertical"] .tab-background:is([selected], [multiselected]) .tab-group-line {
+    height: calc(100% + 6px + var(--tab-block-margin)); // 6px: .tab-group-line's inset-block size
+    left: calc(-1 * var(--tab-inner-inline-margin));
+    transform: translateY(-2px); // .tab-group-line 's inset-inline-start size
+  }
+}
 #tabbrowser-tabs:not([noshadowfortests]) .tab-background:is([selected], [multiselected]) {
   box-shadow: unset !important;
   filter: $ucTabBasicShadow;