#timeline-content, .versionbar {
  width: calc(100% - 20px);
  position: absolute;
  bottom: 6px;
  left: 10px;
  right: 10px;
  // for debugging purposes
  //border: 1px solid black;
}

.timeline {
  background-color: white;
  height: 150px;
  transition: 0.7s ease-in-out;
  opacity: 1;
}

.timelinehide {
  height: 0;
  opacity: 0;
  overflow: hidden;
}

#timelineCanvas {
  border-top: darkgray 1px solid;
}

.btn-timeline {
  border-bottom: none;
  border-color: darkgray;
  border-bottom-right-radius: 0px;
  border-bottom-left-radius: 0px;
  background-color:white;
  padding-bottom: 0.1rem;
  padding-top: 0.1rem;
  display: block;
  position: relative;
  margin-left: auto;
  margin-right: auto;
  top: 1px;
  z-index: 1000;
}

.btn-timeline:focus {
  box-shadow: none!important;
}

.hidetimeline-icon-down {
  transform:rotate(180deg);
}
#timeline-content, .versionbar {
  width: calc(100% - 20px);
  position: absolute;
  bottom: 0px;
  left: 10px;
  right: 10px;
  // for debugging purposes
  //border: 1px solid black;
}

.timeline {
  background-color: white;
  height: 150px;
  transition: 0.7s ease-in-out;
  opacity: 1;
}

.timelinehide {
  height: 0;
  opacity: 0;
  overflow: hidden;
}

#timelineCanvas {
  border-top: darkgray 1px solid;
}

.btn-timeline {
  border-bottom: none;
  border-color: darkgray;
  border-bottom-right-radius: 0px;
  border-bottom-left-radius: 0px;
  background-color:white;
  padding-bottom: 0.1rem;
  padding-top: 0.1rem;
  display: block;
  position: relative;
  margin-left: auto;
  margin-right: auto;
  top: 1px;
  z-index: 490;
}

.btn-timeline:focus {
  box-shadow: none!important;
}

.hidetimeline-icon-down {
  transform:rotate(180deg);
}
Fix timeline position and its button's z-index
Fix timeline position and its button's z-index

The timeline is now positioned at the bottom, before the was a small
gap. Furthermore, the timeline button's z-index was reduced, since
it was rendered above the sidebar if the window was small in width.
