render: scrollback position: only count _used_ scrollback lines

When calculating where in the scrollback history we are, we previously
did this against the total number of scrollback lines. I.e. the
`scrollback.lines` setting in `footrc`.

Now, we count only the used/allocated scrollback lines.

Note that the initial indicator position might still seem to start a
bit high up, if the number of used scrollback lines is low. This is
because we use the *top* of the screen for the current position. Thus,
we'll never be at the bottom (except for the special case when
we're *really* at the bottom).

CHANGELOG.md

@@ -62,6 +62,8 @@
 * Default `--server` socket path to use `$WAYLAND_DISPLAY` instead of
   `$XDG_SESSION_ID` (https://codeberg.org/dnkl/foot/issues/55).
 * Trailing empty cells are no longer highlighted in mouse selections.
+* Scrollback position indicator is now based on the number of _used_
+  scrollback lines, instead of the _total_ number of scrollback lines.
 
 
 ### Fixed

render.c

@@ -1399,14 +1399,22 @@ render_scrollback_position(struct terminal *term)
 
     /* Find absolute row number of the scrollback start */
     int scrollback_start = term->grid->offset + term->rows;
-    while (term->grid->rows[scrollback_start & (term->grid->num_rows - 1)] == NULL)
+    int empty_rows = 0;
+    while (term->grid->rows[scrollback_start & (term->grid->num_rows - 1)] == NULL) {
         scrollback_start++;
+        empty_rows++;
+    }
 
     /* Rebase viewport against scrollback start (so that 0 is at
      * the beginning of the scrollback) */
     int rebased_view = term->grid->view - scrollback_start + term->grid->num_rows;
     rebased_view &= term->grid->num_rows - 1;
 
+    /* How much of the scrollback is actually used? */
+    int populated_rows = term->grid->num_rows - empty_rows;
+    assert(populated_rows > 0);
+    assert(populated_rows <= term->grid->num_rows);
+
     /*
      * How far down in the scrollback we are.
      *
@@ -1414,9 +1422,9 @@ render_scrollback_position(struct terminal *term)
      *  100% -> at the bottom, i.e. where new lines are inserted
      */
     double percent =
-        rebased_view + term->rows == term->grid->num_rows
+        rebased_view + term->rows == populated_rows
         ? 1.0
-        : (double)rebased_view / term->grid->num_rows;
+        : (double)rebased_view / populated_rows;
 
     wchar_t _text[64];
     const wchar_t *text = _text;