I added a way to transform layers with the mouse by pressing the 'G', 'R' or 'S' keys to grab, rotate and scale the selected layers in a similar fashion to blender. I made shift slow the transformation speed by a tenth and ctrl to snap the transformation to intervals. I also allowed users to type in the value for the transformation with the number keys if they wanted a specific value and constrain their transformation to one axis with the 'x' and 'y' keys. You can try the build at https://714e13e8.graphite-master.pages.dev. This was one of my bigger PRs (Collaborating with pull requests - GitHub) at about 550 lines of code available at github.com/GraphiteEditor/Graphite/pull/356/files.
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I worked closely with @Keavon to refine the UX with things like reversing an operation if you switch directly to a new transform operation, removing the negative when all numbers are deleted and ignoring presses to decimal point when there is already a decimal point.

I recently made the scrollbar functional in graphite - an open source drawing program for the web. This built on TrueDoctor's work of finding the bounding box of artwork.

The scrollbars where difficult to implement because they needed to allow users to scroll infinitly far by reserving a space at the end of the scrollbar whilst also not just sharply stopping as they reached a certain point but slowing down the position change as it got further up the track (as specified by Keavon).

My initial approach with this caused some strange issues like the going back over the artwork when the scrollbar was dragged very far or when zoomed out very far (thanks to Keavon and TrueDoctor for finding these problems).  My initial implementation also caused the scrollbar to stop when the artwork was inside the viewport which combined poorly with the dragging mechanism.

This taught me about planning things out on paper as well as responding to feedback.