Center tagline bands, frame anchors evenly, fill to the window edge, and resize columns from their grip

Every tag band (workspace, column, pane) is centred in its row, so text sits
at one baseline offset. Anchors are inset from the column rule by that same
margin, so they are square and the band frames them on the left, top and bottom;
the pane mark moves to stay centred. When the window is not a whole number of
cells, the bands, rules, spines and scroll thumbs at the right and bottom edges
run on through the leftover pixels.

A column grip dropped short of another column's place now moves the column's
left edge, with a dashed rail preview, sharing setColumnPairWidth with the
border drag. A release still on the grip changes nothing, and a pair too
narrow for two MINW columns is left alone.

PARDES_TEST_PAD adds leftover pixels to a test-mode window and capture.

1 files changed, 9 insertions, 7 deletions

diff --git a/docs/config.md b/docs/config.md
index 27818b5b..ab5e5617 100644
--- a/docs/config.md
+++ b/docs/config.md
@@ -398,16 +398,18 @@ band would overlap its pane body or a neighbour instead of leaving the body
 grid stable. `Config` reports the active percentage. The browser applies the
 same compiled percentage to its DOM glyphs but has no runtime setter.
 
-Both native GUIs join the reduced-height global and pane tagline bands with
-`gui_topbar_pane_border_px` physical pixels. Set it to zero for a direct join.
+Both native GUIs separate the reduced-height global and pane tagline bands with
+a `gui_topbar_pane_border_px` physical-pixel rule. Set it to zero to leave it out.
 `gui_topbar_pane_border_rgb` can pin an RGB color; its default `null` follows
 the theme's `border` role in SDL (falling back to `scroll_track` for older
-themes), and `scroll_track` in the macOS shell. With `Tagbottom` enabled, a tagline
-on the final grid row is bottom-aligned so the same unused half-band does not
-show beneath it. The rule itself lives in the core
+themes), and `scroll_track` in the macOS shell. Every band is centred in its
+row, so the workspace, column and pane tag text share one baseline offset and
+the anchors get the same margin above, below and to the left. With `Tagbottom`
+enabled, a tagline on the final grid row is bottom-aligned so the unused
+half-band does not show beneath it. The rule lives in the core
 (`pardes.taglineBandOffset`), and the macOS shell reaches it over the C ABI
-rather than keeping its own copy — it had one, it only ever centred, and the
-seam that leaves between the two bands is what this setting exists to close.
+rather than keeping its own copy. When the window is not a whole number of cells,
+bands and rules at the right and bottom edges run on through the leftover pixels.
 
 What happens to a codepoint the chosen face has no glyph for differs by shell.
 The SDL GUI falls back through a chain it builds itself: embedded Adwaita