tab bar: a sidebar that fits the terminal, and gives up the edge last

`tab_bar_width` was a reservation: the configured columns came off the
screen whatever the screen was, and the only fallback was all the way back
to the top row. On a narrow terminal that is chrome displacing the thing it
is chrome for, and the jump to the top moves every target you were aiming
at.

It is a ceiling now. app_tab_bar_cols takes the app, because the answer
depends on the terminal and not only on the config: held to the columns
left after a pane's collapse width, and to a third of the screen, and
narrowed rather than dropped. TAB_BAR_FIT (14) is the narrowest width this
code will invent on its own -- a config that asks for less is taken at its
word -- and 0 means there is no room for a sidebar at all, which is the one
case that still falls back on top.

Height counts too, which it did not before: a sidebar spends a row per tab,
so a wide but short terminal runs out of list before it runs out of screen,
and a tab with no row is not clipped, it is unreachable. tab_bar_min_rows
is that floor. Not when the top strip could not seat them either -- moving
them would gain nothing, so the side you configured stands.

All of it is re-derived per frame, like the layout, so a resize moves the
bar and nothing remembers that it did.

test_tab_bar also stops depending on where the suite is run from. An
unnamed tab borrows the basename of its focused pane's cwd, so eight checks
read `tests` under `make test` (which cds there) and `slosh` from the
repository root, where they failed. They are about padding, alignment and
the index, not about that derivation, so they name the tab first -- by id,
since `set-name` takes 0 for the focused *pane* only. 147 checks, green
from either directory.

1 files changed, 6 insertions, 3 deletions

diff --git a/include/app_internal.h b/include/app_internal.h
index 40d169a..c6769d2 100644
--- a/include/app_internal.h
+++ b/include/app_internal.h
@@ -489,10 +489,13 @@ void walk_all(app_t *a, leaf_fn fn, void *ud);
 uint16_t eff_gap(split_dir_t dir);
 rect_t app_tab_area(app_t *a);
 /* Where the tab bar lands this frame (TAB_BAR_*), and how many columns a
- * sidebar takes. The side is re-derived per frame: a screen too narrow for
- * the sidebar falls back to the top row. */
+ * sidebar takes. Both are re-derived per frame from the terminal's size: a
+ * sidebar narrows to fit a small screen, and one that can no longer seat
+ * either the panes or its own list of tabs falls back to the top row. The
+ * columns are 0 when there is no room for a sidebar at all; pass a NULL app
+ * to ask only what the config wants. */
 int app_tab_bar_side(const app_t *a);
-uint16_t app_tab_bar_cols(void);
+uint16_t app_tab_bar_cols(const app_t *a);
 void close_leaf(app_t *a, node_t *leaf);
 node_t *first_leaf_of(node_t *n);
 void focus_dir(app_t *a, int dx, int dy);