docs: file collapsible-waybar-sides spec to assets, close task

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+#+TITLE: Collapsible waybar — spike findings (mechanism)
+#+DATE: 2026-06-18
+
+* Question
+Which mechanism actually collapses a waybar side to a base set, given the right
+side is a mix of native modules (group/sysmonitor, pulseaudio, pulseaudio#mic,
+idle_inhibitor, tray) and custom exec modules?
+
+* Method
+Two transient waybar instances against /tmp copies of the live config, captured
+with grim (live bar briefly down, restored after). Variants in this dir:
+- spike-style-csshide.css : option (c) — CSS-hide the native modules
+  (min-width:0; padding:0; margin:0; opacity:0) on #sysmonitor #pulseaudio
+  #idle_inhibitor.
+- spike-config-collapsed.json : option (b) — modules-right rewritten to the base
+  set [tray, custom/date, custom/worldclock].
+
+* Result
+- *CSS-hide (option c): FAILS.* sysmonitor and pulseaudio rendered invisible but
+  held their space — a gap remained where they were, no reflow. GTK3 has no
+  =display:none=, and opacity/zero-size leaves the label's intrinsic width. The
+  right side ends up ragged and half-collapsed, not narrowed. Not viable for the
+  native modules.
+- *Config-swap (option b): WORKS.* The collapsed config reflowed the right side
+  tight to tray + date, everything else fully gone, no gaps. Hides native and
+  custom modules alike. Tray icons survived the swap.
+
+* Decision
+Mechanism is config-swap + =killall -SIGUSR2 waybar= (reload), NOT the state-file
++ CSS approach the original task leaned toward. The original "heavy" label on
+option (b) is the cost of a full reload (brief flicker, module state resets, tray
+re-registers) — acceptable, and the only approach that actually collapses a mixed
+module set.
+
+* Implications for the spec
+- Don't maintain two static configs (drift-prone). A toggle script rewrites the
+  active config's modules-left / modules-right between the full set and the base
+  set, then SIGUSR2. Base sets defined once; collapsed set is the base set, full
+  set is restored from the canonical module list.
+- Per-side state in $XDG_RUNTIME_DIR; the arrow module reads it to pick its
+  direction. Arrow lives IN the base set (always visible, it's the expand control).
+- Reload cost is per-toggle, not idle — fine for a click action.