Document base cell better

Improve the documentation for ncplane_set_base_cell() and
ncplane_set_base(). Go by render.c's authority (all three
channels are independently checked for transparency, and
replaced with the base cell if so--egc, fg, and bg). Add a
BASE CELL section to notcurses_plane.3. Check for, and reject,
an nccell which is secondary among a multicolumn EGC.

Closes #1238.

USAGE.md

@@ -858,17 +858,17 @@ void ncplane_on_styles(struct ncplane* n, unsigned stylebits);
 // Remove the specified styles from the ncplane's existing spec.
 void ncplane_off_styles(struct ncplane* n, unsigned stylebits);
 
-// Set the ncplane's base cell to this cell. It will be used for purposes of
-// rendering anywhere that the ncplane's gcluster is 0. Erasing the ncplane
-// does not reset the base cell; this function must be called with a zero 'c'.
-int ncplane_set_base_cell(struct ncplane* ncp, const nccell* c);
-
-// Set the ncplane's base cell to this cell. It will be used for purposes of
-// rendering anywhere that the ncplane's gcluster is 0. Erasing the ncplane
-// does not reset the base cell; this function must be called with an empty
-// 'egc'. 'egc' must be a single extended grapheme cluster.
-int ncplane_set_base(struct ncplane* ncp, const char* egc,
-                     uint32_t styles, uint64_t channels);
+// Set the ncplane's base nccell to 'c'. The base cell is used for purposes of
+// rendering anywhere that the ncplane's gcluster is 0. Note that the base cell
+// is not affected by ncplane_erase(). 'c' must not be a secondary cell from a
+// multicolumn EGC.
+int ncplane_set_base_cell(struct ncplane* n, const nccell* c);
+
+// Set the ncplane's base nccell. It will be used for purposes of rendering
+// anywhere that the ncplane's gcluster is 0. Note that the base cell is not
+// affected by ncplane_erase(). 'egc' must be an extended grapheme cluster.
+int ncplane_set_base(struct ncplane* n, const char* egc,
+                     uint32_t stylemask, uint64_t channels);
 
 // Extract the ncplane's base cell into 'c'. The reference is invalidated if
 // 'ncp' is destroyed.

doc/man/man3/notcurses_plane.3.md

@@ -253,6 +253,21 @@ invoked following resizing of its parent's plane. If it returns non-zero, the
 resizing cascade terminates, returning non-zero. Otherwise, resizing proceeds
 recursively.
 
+## Base cells
+
+Each plane has a base cell, initialized to all zeroes. When rendering, the
+cells of the plane are examined in turn. Each cell has three independent
+rendering elements--its EGC, its foreground channel, and its background
+channel. Any default channel is replaced with the corresponding channel from
+that plane's base cell. **ncplane_erase** has no effect on the base cell.
+Calling **ncplane_erase** on a plane whose base cell is a purple 'A' results
+(for rendering purposes) in a plane made up entirely of purple 'A's.
+
+**ncplane_set_base_cell** uses the **nccell** ***c*** (which must be bound to
+the **ncplane** ***ncp***, and must be the first **nccell** of a multicolumn
+sequence) to set the base cell. **ncplane_set_base** does the same with
+***egc***, ***stylemask***, and ***channels***.
+
 ## Piles
 
 A single **notcurses** context is made up of one or more piles. A pile is a

include/notcurses/notcurses.h

@@ -1276,15 +1276,15 @@ ncplane_resize_simple(struct ncplane* n, int ylen, int xlen){
 // the standard plane.
 API int ncplane_destroy(struct ncplane* n);
 
-// Set the ncplane's base nccell to this nccell. It will be used for purposes
-// of rendering anywhere that the ncplane's gcluster is 0. Erasing the ncplane
-// does not reset the base cell; this function must be called with a zero 'c'.
+// Set the ncplane's base nccell to 'c'. The base cell is used for purposes of
+// rendering anywhere that the ncplane's gcluster is 0. Note that the base cell
+// is not affected by ncplane_erase(). 'c' must not be a secondary cell from a
+// multicolumn EGC.
 API int ncplane_set_base_cell(struct ncplane* n, const nccell* c);
 
-// Set the ncplane's base nccell to this cell. It will be used for purposes of
-// rendering anywhere that the ncplane's gcluster is 0. Erasing the ncplane
-// does not reset the base cell; this function must be called with an empty
-// 'egc'. 'egc' must be a single extended grapheme cluster.
+// Set the ncplane's base nccell. It will be used for purposes of rendering
+// anywhere that the ncplane's gcluster is 0. Note that the base cell is not
+// affected by ncplane_erase(). 'egc' must be an extended grapheme cluster.
 API int ncplane_set_base(struct ncplane* n, const char* egc,
                          uint32_t stylemask, uint64_t channels);
 

src/lib/notcurses.c

@@ -1390,6 +1390,9 @@ int ncplane_set_bg_palindex(ncplane* n, int idx){
 }
 
 int ncplane_set_base_cell(ncplane* ncp, const nccell* c){
+  if(cell_wide_right_p(c)){
+    return -1;
+  }
   return cell_duplicate(ncp, &ncp->basecell, c);
 }