scrollbar: deprecate min-slider-length style property

In favor of min-height/min-width, like we did for other similar
properties.

gtk/gtkscrollbar.c

@@ -30,6 +30,7 @@
 
 #include "gtkadjustment.h"
 #include "gtkintl.h"
+#include "gtkorientable.h"
 #include "gtkprivate.h"
 
 
@@ -92,6 +93,14 @@ gtk_scrollbar_class_init (GtkScrollbarClass *class)
 
   widget_class->style_updated = gtk_scrollbar_style_updated;
 
+  /**
+   * GtkScrollbar:min-slider-length:
+   *
+   * Minimum length of scrollbar slider.
+   *
+   * Deprecated: 3.20: Use min-height/min-width CSS properties on the slider
+   *   element instead. The value of this style property is ignored.
+   */
   gtk_widget_class_install_style_property (widget_class,
 					   g_param_spec_int ("min-slider-length",
 							     P_("Minimum Slider Length"),
@@ -99,7 +108,7 @@ gtk_scrollbar_class_init (GtkScrollbarClass *class)
 							     0,
 							     G_MAXINT,
 							     21,
-							     GTK_PARAM_READABLE));
+							     GTK_PARAM_READABLE|G_PARAM_DEPRECATED));
 
   gtk_widget_class_install_style_property (widget_class,
 					   g_param_spec_boolean ("fixed-slider-length",
@@ -148,9 +157,14 @@ gtk_scrollbar_update_style (GtkScrollbar *scrollbar)
   gboolean has_a, has_b, has_c, has_d;
   GtkRange *range = GTK_RANGE (scrollbar);
   GtkWidget *widget = GTK_WIDGET (scrollbar);
+  GtkCssGadget *slider_gadget = gtk_range_get_slider_gadget (range);
+
+  gtk_css_gadget_get_preferred_size (slider_gadget,
+                                     gtk_orientable_get_orientation (GTK_ORIENTABLE (scrollbar)), -1,
+                                     &slider_length, NULL,
+                                     NULL, NULL);
 
   gtk_widget_style_get (widget,
-                        "min-slider-length", &slider_length,
                         "fixed-slider-length", &fixed_size,
                         "has-backward-stepper", &has_a,
                         "has-secondary-forward-stepper", &has_b,

gtk/theme/Adwaita/_common.scss

@@ -2008,9 +2008,6 @@ scrollbar {
   @at-root * {
     -GtkScrollbar-has-backward-stepper: false;
     -GtkScrollbar-has-forward-stepper: false;
-    -GtkScrollbar-min-slider-length: 42; // minimum size for the slider.
-                                         // sadly can't be in '.slider'
-                                         // where it belongs
   }
 
   $_slider_margin: 3px;
@@ -2106,6 +2103,7 @@ scrollbar {
   &.vertical {
     slider {
       margin-left: 1px + $_slider_margin;
+      min-height: 42px;
       min-width: 6px;
 
       &:dir(rtl) {
@@ -2137,6 +2135,7 @@ scrollbar {
     slider {
       margin-top: 1px + $_slider_margin;
       min-height: 6px;
+      min-width: 42px;
     }
 
     &.fine-tune slider { margin-top: 1px + $_slider_fine_tune_margin; }

gtk/theme/Adwaita/gtk-contained-dark.css

@@ -2220,8 +2220,7 @@ notebook > stack:not(:only-child) {
  **************/
 * {
   -GtkScrollbar-has-backward-stepper: false;
-  -GtkScrollbar-has-forward-stepper: false;
-  -GtkScrollbar-min-slider-length: 42; }
+  -GtkScrollbar-has-forward-stepper: false; }
 scrollbar button {
   border: none; }
 scrollbar.vertical button.down {
@@ -2275,6 +2274,7 @@ scrollbar.fine-tune slider {
   margin: 4px; }
 scrollbar.vertical slider {
   margin-left: 4px;
+  min-height: 42px;
   min-width: 6px; }
   scrollbar.vertical slider:dir(rtl) {
     margin-left: 3px;
@@ -2291,7 +2291,8 @@ scrollbar.vertical trough {
     border-right-style: solid; }
 scrollbar.horizontal slider {
   margin-top: 4px;
-  min-height: 6px; }
+  min-height: 6px;
+  min-width: 42px; }
 scrollbar.horizontal.fine-tune slider {
   margin-top: 5px; }
 scrollbar.horizontal trough {

gtk/theme/Adwaita/gtk-contained.css

@@ -2230,8 +2230,7 @@ notebook > stack:not(:only-child) {
  **************/
 * {
   -GtkScrollbar-has-backward-stepper: false;
-  -GtkScrollbar-has-forward-stepper: false;
-  -GtkScrollbar-min-slider-length: 42; }
+  -GtkScrollbar-has-forward-stepper: false; }
 scrollbar button {
   border: none; }
 scrollbar.vertical button.down {
@@ -2285,6 +2284,7 @@ scrollbar.fine-tune slider {
   margin: 4px; }
 scrollbar.vertical slider {
   margin-left: 4px;
+  min-height: 42px;
   min-width: 6px; }
   scrollbar.vertical slider:dir(rtl) {
     margin-left: 3px;
@@ -2301,7 +2301,8 @@ scrollbar.vertical trough {
     border-right-style: solid; }
 scrollbar.horizontal slider {
   margin-top: 4px;
-  min-height: 6px; }
+  min-height: 6px;
+  min-width: 42px; }
 scrollbar.horizontal.fine-tune slider {
   margin-top: 5px; }
 scrollbar.horizontal trough {