why sometime the bar disappear???

[yuyang-ok]

Describe the bug

why sometime the bar disappear???

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To Reproduce

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1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

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Expected behavior

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Additional context

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[yuyang-ok]

my css file

.Resizer {
  background: #000;
  opacity: 0.2;
  z-index: 1;
  -moz-box-sizing: border-box;
  -webkit-box-sizing: border-box;
  box-sizing: border-box;
  -moz-background-clip: padding;
  -webkit-background-clip: padding;
  background-clip: padding-box;
}

.Resizer:hover {
  -webkit-transition: all 2s ease;
  transition: all 2s ease;
}

.Resizer.horizontal {
  height: 15px;
  margin: -5px 0;
  border-top: 5px solid rgba(255, 255, 255, 0);
  border-bottom: 5px solid rgba(255, 255, 255, 0);
  cursor: row-resize;
  width: 100%;
  background-color: rgb(71, 71, 71);
}

.Resizer.horizontal:hover {
  border-top: 5px solid rgba(0, 0, 0, 0.5);
  border-bottom: 5px solid rgba(0, 0, 0, 0.5);
}

.Resizer.vertical {
  width: 15px;
  margin: 0 -5px;
  border-left: 5px solid rgba(255, 255, 255, 0);
  border-right: 5px solid rgba(255, 255, 255, 0);
  cursor: col-resize;
  background-color: rgb(71, 71, 71);
}

.Resizer.vertical:hover {
  border-left: 5px solid rgba(0, 0, 0, 0.5);
  border-right: 5px solid rgba(0, 0, 0, 0.5);
}

.Resizer.disabled {
  cursor: not-allowed;
}

.Resizer.disabled:hover {
  border-color: transparent;
}