message: Only allow history selection when on the first or last line
  (of a multi-line or otherwise message in the editor)
diff: @@ -593,12 +593,36 @@ export default class MessageComposerInput extends React.Component {
  };
  onVerticalArrow = (e, up) => {
  - e.preventDefault();
  // Select history only if we are not currently auto-completing
  if (this.autocomplete.state.completionList.length === 0) {
  - return this.selectHistory(up);
  + // Don't go back in history if we're in the middle of a multi-line message
  + const selection = this.state.editorState.getSelection();
  + const blockKey = selection.getStartKey();
  + const firstBlock = this.state.editorState.getCurrentContent().getFirstBlock();
  + const lastBlock = this.state.editorState.getCurrentContent().getLastBlock();
  +
  + const selectionOffset = selection.getAnchorOffset();
  + let canMoveUp = false;
  + let canMoveDown = false;
  + if (blockKey === firstBlock.getKey()) {
  + const textBeforeCursor = firstBlock.getText().slice(0, selectionOffset);
  + canMoveUp = textBeforeCursor.indexOf('\n') === -1;
  + }
  +
  + if (blockKey === lastBlock.getKey()) {
  + const textAfterCursor = lastBlock.getText().slice(selectionOffset);
  + canMoveDown = textAfterCursor.indexOf('\n') === -1;
  + }
  +
  + if ((up && !canMoveUp) || (!up && !canMoveDown)) return;
  +
  + const selected = this.selectHistory(up);
  + if (selected) {
  + // We're selecting history, so prevent the key event from doing anything else
  + e.preventDefault();
  + }
  } else {
  - return this.moveAutocompleteSelection(up);
  + this.moveAutocompleteSelection(up);
  }
  };
  