Key background inherits panel background

NEWS.md

@@ -1,5 +1,9 @@
 # ggplot2 (development version)
 
+* The `legend.key` theme element is set to inherit from the `panel.background`
+  theme element. The default themes no longer set the `legend.key` element.
+  This causes a visual change with the default `theme_gray()` (#5549).
+
 * Lines where `linewidth = NA` are now dropped in `geom_sf()` (#5204).
 
 * New `guide_axis_logticks()` can be used to draw logarithmic tick marks as

R/theme-defaults.R

@@ -172,7 +172,7 @@ theme_grey <- function(base_size = 11, base_family = "",
     legend.spacing.x =    NULL,
     legend.spacing.y =    NULL,
     legend.margin =      margin(half_line, half_line, half_line, half_line),
-    legend.key =         element_rect(fill = "grey95", colour = NA),
+    legend.key =         NULL,
     legend.key.size =    unit(1.2, "lines"),
     legend.key.height =  NULL,
     legend.key.width =   NULL,
@@ -266,8 +266,6 @@ theme_bw <- function(base_size = 11, base_family = "",
       panel.grid.minor = element_line(linewidth = rel(0.5)),
       # contour strips to match panel contour
       strip.background = element_rect(fill = "grey85", colour = "grey20"),
-      # match legend key to background
-      legend.key       = element_rect(fill = "white", colour = NA),
 
       complete = TRUE
     )
@@ -340,9 +338,6 @@ theme_light <- function(base_size = 11, base_family = "",
       # match axes ticks thickness to gridlines and colour to panel border
       axis.ticks       = element_line(colour = "grey70", linewidth = rel(0.5)),
 
-      # match legend key to panel.background
-      legend.key       = element_rect(fill = "white", colour = NA),
-
       # dark strips with light text (inverse contrast compared to theme_grey)
       strip.background = element_rect(fill = "grey70", colour = NA),
       strip.text       = element_text(
@@ -382,9 +377,6 @@ theme_dark <- function(base_size = 11, base_family = "",
       # match axes ticks thickness to gridlines
       axis.ticks       = element_line(colour = "grey20", linewidth = rel(0.5)),
 
-      # match legend key to panel.background
-      legend.key       = element_rect(fill = "grey50", colour = NA),
-
       # dark strips with light text (inverse contrast compared to theme_grey)
       strip.background = element_rect(fill = "grey15", colour = NA),
       strip.text       = element_text(
@@ -442,9 +434,6 @@ theme_classic <- function(base_size = 11, base_family = "",
       # show axes
       axis.line      = element_line(colour = "black", linewidth = rel(1)),
 
-      # match legend key to panel.background
-      legend.key       = element_blank(),
-
       # simple, black and white strips
       strip.background = element_rect(fill = "white", colour = "black", linewidth = rel(2)),
       # NB: size is 1 but clipped, it looks like the 0.5 of the axes
@@ -586,7 +575,7 @@ theme_test <- function(base_size = 11, base_family = "",
     legend.spacing.x =   NULL,
     legend.spacing.y =   NULL,
     legend.margin =      margin(0, 0, 0, 0, "cm"),
-    legend.key =         element_rect(fill = "white", colour=NA),
+    legend.key =         NULL,
     legend.key.size =    unit(1.2, "lines"),
     legend.key.height =  NULL,
     legend.key.width =   NULL,

R/theme-elements.R

@@ -497,7 +497,7 @@ el_def <- function(class = NULL, inherit = NULL, description = NULL) {
   legend.spacing      = el_def("unit"),
   legend.spacing.x     = el_def(c("unit", "rel"), "legend.spacing"),
   legend.spacing.y     = el_def(c("unit", "rel"), "legend.spacing"),
-  legend.key          = el_def("element_rect", "rect"),
+  legend.key          = el_def("element_rect", "panel.background"),
   legend.key.height   = el_def(c("unit", "rel"), "legend.key.size"),
   legend.key.width    = el_def(c("unit", "rel"), "legend.key.size"),
   legend.text         = el_def("element_text", "text"),