code: function queryPreviewProviders(editor, pos, token) {
          var line = editor.document.getLine(pos.line);
  
          // FUTURE: Support plugin providers. For now we just hard-code...
          var popover = colorAndGradientPreviewProvider(editor, pos, token, line) ||
                        imagePreviewProvider(editor, pos, token, line);
  
          if (popover) {
              // Providers return just { start, end, content, ?onShow, xpos, ytop, ybot }
              popover.visible = false;
              popover.editor  = editor;
              return popover;
          }
  
          return null;
      }
docs: Preview hide/show logic ------------------------------------------------ 
  Returns a 'ready for use' popover state object:
  { visible: false, editor, start, end, content, ?onShow, xpos, ytop, ybot }
  Lacks only hoverTimer (supplied by handleMouseMove()) and marker (supplied by showPreview()).