discourse/app/assets/stylesheets/common/rich-editor/rich-editor.scss

.ProseMirror-container {
  margin: 0;
  overscroll-behavior: contain;
  overflow-anchor: none;
  flex-direction: column;
  overflow-y: auto;
  overflow-x: hidden;
  display: flex;
  height: 100%;
}

.ProseMirror {
  outline: 0;
  padding: 0 0.625rem;

  > div:first-child,
  > details:first-child {
    // This is hacky, but helps having the leading gapcursor at the right position
    &.ProseMirror-gapcursor {
      position: relative;
      display: block;
    }
    margin-top: 0.5rem;
  }

  h1,
  h2,
  h3,
  h4,
  h5,
  h6 {
    margin: 30px 0 10px;

    &:first-child {
      margin-top: 10px;
    }
  }

  h1 {
    font-size: var(--font-up-3-rem);
  }

  h2 {
    font-size: var(--font-up-2-rem);
  }

  h3 {
    font-size: var(--font-up-1-rem);
  }

  img {
    display: inline-block;
    height: auto;
    max-width: 100%;

    &[data-placeholder="true"] {
      animation: placeholder 1.5s infinite;

      @keyframes placeholder {
        0% {
          opacity: 0.6;
        }

        50% {
          opacity: 0.4;
        }

        100% {
          opacity: 0.6;
        }
      }
    }
  }

  ul,
  ol {
    padding-left: 1.25em;

    &[data-tight="true"] > li > p {
      margin-top: 0;
      margin-bottom: 0;
    }
  }

  p {
    line-height: 1.5;

    &:first-child {
      margin-top: 0.59rem;
    }
  }

  p[data-placeholder]::before {
    pointer-events: none;
    position: absolute;
    padding-top: 2px;
    padding-right: 0.5rem;
    content: attr(data-placeholder);
    color: var(--primary-400);
    line-height: 1.1;
  }

  del {
    background-color: var(--danger-low);
  }

  ins {
    background-color: var(--success-low);
  }

  mark {
    background-color: var(--highlight);
  }

  td {
    padding: 3px 3px 3px 0.5em;
  }

  th {
    padding-bottom: 2px;
    font-weight: bold;
    color: var(--primary);
  }

  kbd {
    // I believe this shouldn't be `inline-flex` in posts either (test with emojis before/after text to see why),
    // but overriding just for the editor for now
    display: inline;
    padding-top: 0.2rem;
  }

  .onebox-wrapper {
    white-space: normal;
  }

  .code-block {
    position: relative;
  }

  .code-language-select {
    position: absolute;
    right: 0.25rem;
    top: -0.6rem;
    border: 1px solid var(--primary-low);
    border-radius: var(--d-border-radius);
    background-color: var(--primary-very-low);
    color: var(--primary-medium);
    font-size: var(--font-down-1-rem);
  }

  .html-block {
    position: relative;
    border: 1px dashed var(--primary-low-mid);

    &::after {
      position: absolute;
      right: 0;
      top: 0;
      content: "HTML";
      font-size: var(--font-down-2-rem);
      color: var(--primary-low-mid);
      z-index: 1;
    }
  }
}

/*********************************************************
 Section below from prosemirror-view/style/prosemirror.css
 ********************************************************/

.ProseMirror {
  position: relative;
}

.ProseMirror {
  word-wrap: break-word;
  white-space: break-spaces;
}

.ProseMirror pre {
  white-space: pre-wrap;
}

.ProseMirror li {
  position: relative;
}

.ProseMirror-hideselection *::selection {
  background: transparent;
}

.ProseMirror-hideselection {
  caret-color: transparent;
}

/* See https://github.com/ProseMirror/prosemirror/issues/1421#issuecomment-1759320191 */
.ProseMirror [draggable][contenteditable="false"] {
  user-select: text;
}

.ProseMirror-selectednode {
  outline: 2px solid #8cf;
}

/* Make sure li selections wrap around markers */
li.ProseMirror-selectednode {
  outline: none;
}

li.ProseMirror-selectednode::after {
  content: "";
  position: absolute;
  left: -32px;
  right: -2px;
  top: -2px;
  bottom: -2px;
  border: 2px solid #8cf;
  pointer-events: none;
}

/* Protect against generic img rules */
.ProseMirror-separator {
  display: inline !important;
  border: none !important;
  margin: 0 !important;
}

/************************************************************
 Section below from prosemirror-gapcursor/style/gapcursor.css
 ***********************************************************/

.ProseMirror-gapcursor {
  display: none;
  pointer-events: none;
  position: absolute;
}

.ProseMirror-gapcursor::after {
  content: "";
  display: block;
  position: absolute;
  top: -2px;
  width: 20px;
  border-top: 1px solid var(--primary);
  animation: ProseMirror-cursor-blink 1.1s steps(2, start) infinite;
}

@keyframes ProseMirror-cursor-blink {
  to {
    visibility: hidden;
  }
}

.ProseMirror-focused .ProseMirror-gapcursor {
  display: block;
}