Get Viewport Lines and Columns in CodeMirror

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CodeMirror is an amazing utility for presenting code in a browser environment.  Syntax highlighting, widgets, and a number of advanced functions make it a unique, useful tool.  When using CodeMirror inside the Firefox DevTools debugger, I found that adding hundreds of column breakpoint widgets to very long lines of code really killed performance, and I sure as hell can’t give you all a horrible experience while you’re debugging your JavaScript.

I wanted to get fancy to ensure performance was good, so I decided to tinker around with only displaying column breakpoint widgets that appeared in the viewport.   To do that, I needed to calculate the start line, start column, end line, and end column of the CodeMirror editor’s contents, some of which didn’t appear to be provided within methods of CodeMirror.

My experimentation led to me to a solution I’m quite happy with; the code is clean, the performance is good, and the method has been incredibly reliable.  Here it is:

function getLocationsInViewport(editor) {
  const charWidth = editor.defaultCharWidth();
  const scrollArea = editor.getScrollInfo();
  const { scrollLeft } = editor.doc;
  const rect = editor.getWrapperElement().getBoundingClientRect();

  const topVisibleLine = editor.lineAtHeight(rect.top, "window");
  const bottomVisibleLine = editor.lineAtHeight(
    rect.bottom,
    "window"
  );

  const leftColumn = Math.floor(scrollLeft > 0 ? scrollLeft / charWidth : 0);
  const rightPosition = scrollLeft + (scrollArea.clientWidth - 30);
  const rightColumn = Math.floor(rightPosition / charWidth);
   return {
    start: {
      line: topVisibleLine,
      column: leftColumn
    },
    end: {
      line: bottomVisibleLine,
      column: rightColumn
    }
  };
}

CodeMirror does provide easy methods for getting the start and end lines in viewport (lineAtHeight) but there’s not a similar functionality for column. I opted to get the scrollLeft position of CodeMirror’s scroller, then use the default character width and other dimensions to get the approximate column at that position.  My user testing found this method to be very reliable, either at the exact character or one character off (likely due to subpixel math).

I’ve never proclaimed to be the best developer in the world (I’m far from it) but being clever to find solutions to interesting problems is something that I’ve always been proud of.

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