Tree Style Tabs

readme.md

Based on the great work from theprojectsomething: https://gist.github.com/theprojectsomething/6813b2c27611be03e67c78d936b0f760

Select "Horizontal Tab Bar" and "Collapse Tree Style Tab" in the "Tab Bar Style" Select "Optimize browser for Vertical Tab Bar" to hide the Horizontal tab bar

Under the advanced tab in the Tree Style Tab options past in the below theme.css

The main points that might need tweaking for you would be:

/* This value should be updated here and in the userChrome.css */
  --tst-sidepanel-hide-delay: .5s;
}

which will determine how quickly the icons shift as you mouseover.

/* Push tab labels slightly to the right so they're completely hidden in collapsed state, but still look fine while expanded. */
.tab .label {
  margin-left: 2em;
}

Will determine how much spacing to the right the name of the website gets. You will generally want this far enough that it's not showing when the tab bar is not being moused over

tree-style-tabs-theme.css

:root {
  /* This value should be updated here and in the userChrome.css */
  --tst-sidepanel-hide-delay: .5s;
}

/* Hide border on tab bar, force its state to 'scroll', adjust margin-left for width of scrollbar. */
#tabbar {
  border: 0;
  scrollbar-width: none;
  overflow: scroll !important;
  margin-top:  0 !important;
}

/* resolve extra space for scrollbar (scrollbar is hidden by this script) */
.on-scrollbar-area #tabbar {
  --shift-tabs-for-scrollbar-distance: 0px;
}

/* Include 'reveal' animation ... stagers by level */
#tabbar .tab {
  transition: 0.1s margin-top, 0.2s 0.1s visibility;
}
#tabbar tab-item-substance {
  transition: 0.2s 0.1s margin-left;
}


/* delay transitions on open */
#tabbar:not(:hover) tab-item-substance {
  transition-delay: var(--tst-sidepanel-hide-delay);
  margin-left: 0;
}

/* animate twisty reveal */
#tabbar .tab .twisty {
  transition: 0.2s margin;
}

/* general tabs */
.tab {
  background-color: transparent;
}
.tab,
.tab.pinned {
  height: 2.8em;
}

/* Push tab labels slightly to the right so they're completely hidden in collapsed state, but still look fine while expanded. */
.tab .label {
  margin-left: 2em;
}

/* fix closebox */
.tab .closebox {
  margin-left:  0;
}

.tab .counter {
  margin-left:  auto;
  display: inline-block !important;
}

/* Hide .twisty and adjust margins so favicons have 7px on left. */
#tabbar:not(:hover) .tab .twisty {
  visibility: hidden;
  margin-left: -5px;
  transition-delay: var(--tst-sidepanel-hide-delay);
}


/* hide closebox unless needed */
.tab:not(:hover) .closebox {
  visibility: hidden;
}

/* Hide sound playing/muted button. */
.sound-button {
  margin-left: 0;
  display: inline-block;
  visibility: collapse;
}

.tab.audible .sound-button {
  visibility: visible;
  margin-left: 0.25em;
}

.tab:not([data-child-ids]) .counter {
  /* visibility: hidden; */
}

tab-item:not(.subtree-collapsed) .counter {
  visibility: hidden;
} 



/* active tab */
.tab.active {
  background-color: rgba(255, 255, 255, 0.05) !important;
}

tab-item.active .highlighter::before {
  background-color: #fffd !important;
}

.tab:hover,
.tab.active:hover {
  background-color: inherit;
}
.tab.active .label {
  font-weight: bold;
  color: #f4f4f4 !important;
}
.tab .label,
.tab.active .label {
  border-bottom:  1px solid transparent;
}
.tab:hover .label,
.tab.active:hover .label {
  border-bottom:  1px dotted;
  min-width:  0 !important;
  flex-shrink:  1 !important;
  flex-grow:  unset !important;
}

/* pending / unloaded tabs */
.tab.discarded {
  background-color: #1d1d1d;
}
.tab.discarded .label {
  color: #efefefCC !important;
}
.tab.discarded:hover .label {
  color: #efefef !important;
}

/* Adjust style for tab that has sound playing. */
.tab.sound-playing .favicon::after,
.tab.muted .favicon::after {
  content: '🔊';
  z-index: var(--favicon-base-z-index);
  position: absolute;
  font-size: 0.5em;
  bottom: -0.35em;
  right: -0.7em;
}

/* Adjust style for tab that is muted. */
.tab.muted .favicon::after {
  content: '🔇';
}

/* Pinned tabs: */
/* Hide all non-active pinned tabs (these are included in top-bar instead) */
.tab.pinned {
  position: relative;
  max-width: none;
  width: auto;
  top: 0 !important;
  left: 0 !important;
}

.tab.pinned .label,
.tab.pinned .label-content {
  opacity: 1;
  position: unset;
  padding-bottom: 0;
}
.tab.pinned .sound-button {
  position: relative;
  transform: none;
}
.tab.pinned .twisty {
  display: block;
  min-width: none;
  width: auto;
}

here's a screenshot that shows the full button via the dropdown

not sure if this is entirely what's going on, but you can also add margins for the y dimension too, like this:

.newtab-button {
  display: yes;
  visibility: visible;
  margin-left: .25em;
  margin-top: .25em;
}