What

When ruler is turned on, the value read from and the value written into figma.viewport are treated differently.

Repro

figma.showUI(__html__);

figma.ui.onmessage = (msg) => {
  if (msg.type === "set-viewport") {
    figma.viewport.center = {
      x: figma.viewport.center.x,
      y: figma.viewport.center.y,
    };
  }
};

<h2>Bug repro</h2>
<button id="set-viewport">Set viewport</button>

<script>
  document.getElementById("set-viewport").onclick = () => {
    parent.postMessage({ pluginMessage: { type: "set-viewport" } }, "*");
  };
</script>

Bug Implications

I maintain a beloved a plugin that allows user to use a custom device to zoom/pan the canvas. It calculates the x and y difference the viewport needs to travel and apply it as user controls the device. The plugin replies on the accurate behaviour of the figma.viewport API. Here is a related bug my user filed on GitHub.

In addition, it think it’s a fair assumption that read the viewport and immediately writing back to it should not cause a drift, whether the ruler is on or not.