Merge pull request rstudio#1641 from rstudio/fix-plot-rounding

Round brush coordinates

Author: wch

NEWS.md

@@ -62,6 +62,8 @@ in shiny apps. For more info, see the documentation (`?updateQueryString` and `?
 
 * Fixed [#1621](https://github.com/rstudio/shiny/issues/1621): if a `reactiveTimer`'s session was closed before the first time that the `reactiveTimer` fired, then the `reactiveTimer` would not get cleared and would keep firing indefinitely. ([#1623](https://github.com/rstudio/shiny/pull/1623))
 
+* Fixed [#1634](https://github.com/rstudio/shiny/issues/1634): If brushing on a plot causes the plot to redraw, then the redraw could in turn trigger the plot to redraw again and again. This was due to spurious changes in values of floating point numbers. ([#1641](https://github.com/rstudio/shiny/pull/1641))
+
 ### Library updates
 
 * Closed [#1500](https://github.com/rstudio/shiny/issues/1500): Updated ion.rangeSlider to 2.1.6. ([#1540](https://github.com/rstudio/shiny/pull/1540))

inst/www/shared/shiny.js

@@ -1119,6 +1119,9 @@ imageutils.createBrush = function($el, opts, coordmap, expandPixels) {
     // For reversed scales, the min and max can be reversed, so use findBox
     // to ensure correct order.
     state.boundsData = coordmap.findBox(minData, maxData);
+    // Round to 14 significant digits to avoid spurious changes in FP values
+    // (#1634).
+    state.boundsData = mapValues(state.boundsData, val => roundSignif(val, 14));
 
     // We also need to attach the data bounds and panel as data attributes, so
     // that if the image is re-sent, we can grab the data bounds to create a new

inst/www/shared/shiny.js.map

@@ -56,6 +56,17 @@ function padZeros(n, digits) {
   return str;
 }
 
+// Round to a specified number of significant digits.
+function roundSignif(x, digits = 1) {
+  if (digits < 1)
+    throw "Significant digits must be at least 1.";
+
+  // This converts to a string and back to a number, which is inelegant, but
+  // is less prone to FP rounding error than an alternate method which used
+  // Math.round().
+  return parseFloat(x.toPrecision(digits));
+}
+
 // Take a string with format "YYYY-MM-DD" and return a Date object.
 // IE8 and QTWebKit don't support YYYY-MM-DD, but they support YYYY/MM/DD
 function parseDate(dateString) {