Note that painting without a change adds no undo step (#212)

Development.md

@@ -13,7 +13,7 @@ npm run check        # typecheck + lint + format:check + tests, the gate CI runs
 npm run build        # writes dist/ and regenerates index.html
 ./build.sh           # the same under `set -o pipefail`, for callers that pipe the output
 php tools/make-user.php <name> <password>                     # an account; every tool signs in (see Accounts)
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1312 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1313 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result

Tilemaps.md

@@ -91,7 +91,8 @@ layers there is what the click selects.
 Shift-drag still pans the view, whatever the tool.
 
 **One stroke is one undo step**, and so is one fill — a line of twenty cells comes back with one Ctrl+Z, and a stroke
-painted over another tile puts *that* one back, not merely emptiness.
+painted over another tile puts *that* one back, not merely emptiness. A stroke, rectangle or fill
+that changes no cell leaves no undo step at all (#212).
 
 Under **TILES** stands every tile of the project as a grid of pictures with its name below
 (#139), so you see what you paint with. A floor or wall tile shows its lone tile — the four