selection: recognize empty padding cells in a forced linewrap

When printing a multi-column character at the end of the line, and it
doesn't fit, we currently insert a forced line-wrap. This means the
last character(s) on the previous line will be empty, followed by a
multi-column character in the first cell on the next line.

Without special code to handle this, the selection text extraction
code will insert a hard newline, since this is normally the correct
thing to do.

Add a TODO, to consider writing a special place holder value to these
padding cells.

selection.c

@@ -269,15 +269,41 @@ extract_one(struct terminal *term, struct row *row, struct cell *cell,
 {
     struct extract *ctx = data;
 
-    if (ctx->last_row != NULL && row != ctx->last_row &&
-        ((term->selection.kind == SELECTION_NORMAL &&
-          (ctx->last_row->linebreak ||
-           ctx->empty_count > 0 || cell->wc == 0)) ||
-         term->selection.kind == SELECTION_BLOCK))
-    {
-        /* Last cell was the last column in the selection */
-        ctx->buf[ctx->idx++] = L'\n';
-        ctx->empty_count = 0;
+    if (ctx->last_row != NULL && row != ctx->last_row) {
+        /* New row - determine if we should insert a newline or not */
+
+        if (term->selection.kind == SELECTION_NORMAL) {
+            int width = max(1, wcwidth(cell->wc));
+
+            if (width > 1) {
+                /* Heuristict to handle force-wrapped multi-column
+                 * characters */
+
+                /*
+                 * TODO: maybe we should print a placeholder value to
+                 * the empty cells at the end of the line when
+                 * force-wrapping? Then extract() could simply skip
+                 * those cells
+                 */
+                ctx->empty_count -= min(width, ctx->empty_count);
+            }
+
+            else if (ctx->last_row->linebreak ||
+                ctx->empty_count > 0 ||
+                cell->wc == 0)
+            {
+                /* Row has a hard linebreak, or either last cell or
+                 * current cell is empty */
+                ctx->buf[ctx->idx++] = L'\n';
+                ctx->empty_count = 0;
+            }
+        }
+
+        else if (term->selection.kind == SELECTION_BLOCK) {
+            /* Always insert a linebreak */
+            ctx->buf[ctx->idx++] = L'\n';
+            ctx->empty_count = 0;
+        }
     }
 
     if (cell->wc == 0) {

terminal.c

@@ -2399,6 +2399,16 @@ term_print(struct terminal *term, wchar_t wc, int width)
         /* Multi-column character that doesn't fit on current line -
          * force a line wrap */
         term->grid->cursor.lcf = 1;
+
+        /*
+         * TODO: should we insert place holder values in the remaining
+         * cells?  This would allow e.g. text extraction to simply
+         * skip these, instead of trying to recognize a sequence of
+         * empty cells at the end of the line followed by a
+         * multi-column character...
+         *
+         * Might also make text reflow easier, or even more correct.
+         */
     }
 
     print_linewrap(term);