selection: never highlight selected, empty cells

This fixes a regression, where empty cells "between" non-empty
cells (i.e. non-trailing empty cells) sometimes were incorrectly
highlighted.

The idea has always been to highlight exactly those cells that will
get extracted when they’re copied. This means we’ve not highlighted
trailing empty cells, but we _have_ highlighted other empty cells,
since they are converted to spaces when copied (whereas trailing empty
cells are skipped).

fa2d9f8699 changed how a selection is
updated. That is, which cells gets marked as selected, and which ones
gets unmarked.

Since we no longer walk all the cells, but instead work with pixman
regions representing selection diffs, we can no longer determine (with
certainty) which empty cells should be selected and which shouldn’t.

Before this patch (but after
fa2d9f8699), we sometimes incorrectly
highlighted empty cells that should not have been highlighted. This
happened when we’ve first (correctly) highlighted a region of empty
cells, but then shrink the selection such that all those empty cells
should be de-selected.

This patch changes the selection behavior to *never* highlight empty
cells. This fixes the regression, but also means slightly different
behavior, compared to pre-fa2d9f86996467ba33cc381f810ea966a4323381.

The other alternative is to always highlight all empty cells. But,
since I personally like the fact that we’re skipping trailing empty
cells, I prefer the approach taken by this patch.

CHANGELOG.md

@@ -51,6 +51,8 @@
 ### Changed
 
 * Window is now dimmed while in Unicode input mode.
+* Selected empty cells are **never** highlighted as being
+  selected. They used to be, when followed by non-empty cells.
 
 
 ### Deprecated
@@ -61,6 +63,8 @@
 * Crash on buggy compositors (GNOME) that sometimes send pointer-enter
   events with a NULL surface. Foot now ignores these events, and the
   subsequent motion and leave events.
+* Regression: “random” selected empty cells being highlighted as
+  selected when they should not.
 
 
 ### Security

selection.c

@@ -737,8 +737,27 @@ mark_selected_region(struct terminal *term, pixman_box32_t *boxes,
                 row->dirty = true;
 
             for (int c = box->x1, empty_count = 0; c < box->x2; c++) {
-                if (selected && row->cells[c].wc == 0) {
+                if (row->cells[c].wc == 0) {
-                    empty_count++;
+                    /*
+                     * We used to highlight empty cells *if* they were
+                     * followed by non-empty cell(s), since this
+                     * corresponds to what gets extracted when the
+                     * selection is copied (that is, empty cells
+                     * “between” non-empty cells are converted to
+                     * spaces).
+                     *
+                     * However, they way we handle selection updates
+                     * (diffing the “old” selection area against the
+                     * “new” one, using pixman regions), means we
+                     * can’t correctly update the state of empty
+                     * cells. The result is “random” empty cells being
+                     * rendered as selected when they shouldn’t.
+                     *
+                     * “Fix” by *never* highlighting selected empty
+                     * cells (they still get converted to spaces when
+                     * copied, if followed by non-empty cells).
+                     */
+                    /* empty_count++; */
                     continue;
                 }