(perf): measure legend once

NAMESPACE

@@ -68,6 +68,7 @@ importFrom(grDevices,
   dev.list,
   dev.new,
   dev.off,
+  dev.size,
   extendrange,
   gray.colors,
   hcl,

NEWS.md

@@ -313,7 +313,7 @@ not 'at'`. The free-facet code path listed the eligible types by name, so
 
 ### Internals
 
-- Performance improvemnts. (#723 @grantmcdermott)
+- Performance improvements. (#723, #724 @grantmcdermott)
 
 ## v0.7.0
 

R/legend.R

@@ -258,8 +258,14 @@ tinylegend = function(legend_env) {
     legend_env$args[["text.width"]] = NULL
   }
 
-  # Re-measure legend dimensions (device size may have changed on resize)
-  legend_env$dims = measure_fake_legend(legend_env)
+  # Re-measure legend dimensions, but only when the device has actually
+  # changed. draw_legend() already measured on this device during setup, so on
+  # the initial draw the result is still current and a measuring pass costs
+  # about as much as drawing the legend itself. A resize, or a replay onto a
+  # different device, fails the key comparison and forces the re-measure.
+  if (is.null(legend_env$dims) || !identical(legend_env$dims_dev, legend_dev_key())) {
+    legend_env$dims = measure_fake_legend(legend_env)
+  }
 
   # Calculate and apply soma (outer margin adjustment based on legend size)
   # When soma_target is set (multi-legend), use it directly so all legends
@@ -362,6 +368,17 @@ tinylegend = function(legend_env) {
 
 
 # Measure legend dimensions using a fake (non-plotted) legend
+# Identity of the device a legend measurement belongs to. Size alone is not
+# enough: replaying or copying a display list onto a same-sized device with a
+# different backend (dev.copy(), dev.print(), an IDE's plot export) yields
+# different text metrics, so the device itself has to be part of the key.
+# dev.cur() is a named integer, so this captures the backend as well as the
+# device number.
+legend_dev_key = function() {
+  list(dev = dev.cur(), size = dev.size())
+}
+
+
 measure_fake_legend = function(legend_env) {
   fklgnd.args = modifyList(
     legend_env$args,
@@ -389,6 +406,10 @@ measure_fake_legend = function(legend_env) {
     }
   }
 
+  # Record which device this measurement was taken on, so callers can tell
+  # whether a cached result is still valid (see tinylegend()).
+  legend_env$dims_dev = legend_dev_key()
+
   do.call("legend", fklgnd.args)
 }
 
@@ -929,7 +950,7 @@ build_legend_env = function(
 #'   with a legend in the margin.
 #'
 #' @importFrom graphics grconvertX grconvertY rasterImage strheight strwidth xinch
-#' @importFrom grDevices as.raster recordGraphics
+#' @importFrom grDevices as.raster dev.size recordGraphics
 #' @importFrom utils modifyList
 #'
 #' @examples