Fix calculation of theme defaults

**Issue**
At plot time, the calculation of theme elements always fell back
to the default theme. As a consequence, the default theme would
leak into complete themes where some of the elements were
unspecified.

**Solution**
Be more specific when calculating the theme defaults.

**Clean Up**
`theme_void` was insufficiently specified. The solution exposed
missing elements that had to be specified.

Fixes tidyverse#2058
Fixes tidyverse#2079

NEWS.md

@@ -135,6 +135,8 @@
 
 * `position_jitter()` gains a `seed` argument that allows specifying a random seed for reproducible jittering (#1996, @krlmlr).
 
+* Fixed bug where a new complete `theme` may fail to override all elements of the default `theme`.
+  (@has2k1, #2058, #2079)
 
 ### sf
 

R/theme-defaults.r

@@ -388,9 +388,12 @@ theme_void <- function(base_size = 11, base_family = "",
                          ),
     axis.text =        element_blank(),
     axis.title =       element_blank(),
+    axis.ticks.length =  unit(0, "pt"),
+    legend.position =    "right",
     legend.text =        element_text(size = rel(0.8)),
     legend.title =       element_text(hjust = 0),
     strip.text =         element_text(size = rel(0.8)),
+    panel.ontop =        FALSE,
     plot.margin =        unit(c(0, 0, 0, 0), "lines"),
 
     complete = TRUE

R/theme.r

@@ -416,7 +416,14 @@ theme <- function(line,
 
 # Combine plot defaults with current theme to get complete theme for a plot
 plot_theme <- function(x) {
-  defaults(x$theme, theme_get())
+  complete <- attr(x$theme, "complete")
+  if (is.null(complete)) {
+    theme_get()
+  } else if (complete) {
+    x$theme
+  } else {
+    defaults(x$theme, theme_get())
+  }
 }
 
 #' Modify properties of an element in a theme object