Tried to improve on transitions and animations

src/browser/base/content/zen-styles/zen-animations.css

@@ -44,17 +44,6 @@
   }
 }
 
-@keyframes zen-zoom-in {
-  from {
-    transform: scale(0.5);
-    opacity: 0;
-  }
-  to {
-    opacity: 1;
-    transform: scale(1);
-  }
-}
-
 @keyframes zen-deck-fadeIn {
   0% {
     transform: scale(0.9);
@@ -81,13 +70,12 @@
 
 @keyframes zen-sidebar-panel-animation-2 {
   0% {
-    opacity: 0;
-    transform: translateX(-50px) scale3d(0.95, 0.95, 0.95);
+    opacity: 1;
+    transform: translateX(-50px);
   }
 
   100% {
-    opacity: 1;
-    transform: translateX(0) scale3d(1, 1, 1);
+    transform: translateX(0);
   }
 }
 

src/browser/base/content/zen-styles/zen-decks.css

@@ -21,7 +21,7 @@
 }
 
 #tabbrowser-tabpanels[zen-split-view='true'] > [zen-split-anim='true'] {
-  animation: zen-deck-fadeIn 0.2s forwards;
+  animation: zen-deck-fadeIn 0.2s forwards ease-in-out;
 }
 
 #tabbrowser-tabpanels[zen-split-view='true'] .browserSidebarContainer[zen-split-active='true'] {

src/browser/base/content/zen-styles/zen-panel-ui.css

@@ -2,6 +2,6 @@ panel[type='arrow'][animate][animate='open'] {
   animation: zen-jello-animation 0.2s ease-in-out;
 }
 
-panel[type='arrow'][animate][animate='cancel'] {
+panel[type='arrow'][animate]:not([animate='open']) {
   animation: zen-jello-out-animation 0.2s ease-in-out;
 }

src/browser/base/content/zen-styles/zen-tabs/vertical-tabs.css

@@ -83,7 +83,7 @@
     grid-gap: 0 !important;
 
     & .tabbrowser-tab {
-      animation: zen-slide-in 0.2s;
+      animation: zen-slide-in 0.2s ease-in-out;
 
       max-width: unset !important;
       padding: 0 !important;