Stop HUD names ghosting during the battle window shake (#295)

The zone pass composited two copies of the baked canvas on shake
frames: a base copy and the offset copy. The canvas holds window-layer
content (HUD names, the text box), so the vacated strip showed a full
second copy of the enemy name. Draw only the shifted copy and fill the
strip blank, like the hardware revealing empty BG.

src/battle/BattleState.lua

@@ -4006,9 +4006,10 @@ end
 
 -- recolor the grayscale BG canvas per zone; an active BGP fade permutes
 -- the zone palette (the SGB colors the remapped DMG shade).  A window
--- shake draws a second, offset copy over the base one: the color
+-- shake draws only the offset copy: the baked canvas holds the HUDs and
--- regions themselves never move on the SGB, and the vacated strip
+-- text box (window-layer content), so compositing an unshifted copy
--- shows the unshifted BG map like the hardware.
+-- underneath ghosted every name in the vacated strip (#295).  The strip
+-- shows blank color 0 instead, like the hardware revealing empty BG.
 function BattleState:drawZonePass(src, sx, sy)
   local PaletteFX = require("src.render.PaletteFX")
   local shader = PaletteFX.shader()
@@ -4016,13 +4017,17 @@ function BattleState:drawZonePass(src, sx, sy)
   local bgp = self:activeBgp()
   love.graphics.setColor(1, 1, 1, 1)
   love.graphics.setShader(shader)
+  local shaking = sx ~= 0 or sy ~= 0
   for _, z in ipairs(BATTLE_ZONES) do
     PaletteFX.sendColors(shader, PaletteFX.permute(pals[z.pal], bgp))
-    love.graphics.setScissor(z[1] * 8, z[2] * 8,
+    local zx, zy = z[1] * 8, z[2] * 8
-                             (z[3] - z[1] + 1) * 8, (z[4] - z[2] + 1) * 8)
+    local zw, zh = (z[3] - z[1] + 1) * 8, (z[4] - z[2] + 1) * 8
-    love.graphics.draw(src, 0, 0)
+    love.graphics.setScissor(zx, zy, zw, zh)
-    if sx ~= 0 or sy ~= 0 then
+    if shaking then
+      love.graphics.rectangle("fill", zx, zy, zw, zh)
       love.graphics.draw(src, sx, sy)
+    else
+      love.graphics.draw(src, 0, 0)
     end
   end
   love.graphics.setScissor()

tests/run_tests.lua

@@ -2886,6 +2886,47 @@ do
   Font.drawCode, Font.drawBox = origCode, origBox
   eq(drawn, 0, "no current message and no animation draws no text")
 end
+
+-- issue #295: during a window shake the zone pass draws only the shifted
+-- copy; a base copy underneath ghosted the HUD names in the vacated strip
+do
+  local PaletteFX = require("src.render.PaletteFX")
+  local origShaderFn, origSend = PaletteFX.shader, PaletteFX.sendColors
+  local origPermute = PaletteFX.permute
+  local origDraw, origRect = love.graphics.draw, love.graphics.rectangle
+  PaletteFX.shader = function() return nil end
+  PaletteFX.sendColors = function() end
+  PaletteFX.permute = function(p) return p end
+  local draws, fills = {}, 0
+  love.graphics.draw = function(_, x, y) draws[#draws + 1] = { x, y } end
+  love.graphics.rectangle = function(mode) if mode == "fill" then fills = fills + 1 end end
+  local battle = setmetatable({ fx = {} }, BattleState)
+  function battle:sgbBattlePals() return setmetatable({}, { __index = function() return {} end }) end
+  function battle:activeBgp() return nil end
+
+  battle:drawZonePass("canvas", 0, 8)
+  check(#draws > 0, "the zone pass still draws during a shake")
+  local shiftedOnly = true
+  for _, d in ipairs(draws) do
+    if d[1] == 0 and d[2] == 0 then shiftedOnly = false end
+    if d[2] ~= 8 then shiftedOnly = false end
+  end
+  check(shiftedOnly, "a shake draws only the shifted copy, no base copy")
+  check(fills == #draws, "the vacated strip is filled blank per zone")
+
+  draws, fills = {}, 0
+  battle:drawZonePass("canvas", 0, 0)
+  local baseOnly = #draws > 0
+  for _, d in ipairs(draws) do
+    if d[1] ~= 0 or d[2] ~= 0 then baseOnly = false end
+  end
+  check(baseOnly, "no shake draws the single base copy as before")
+  check(fills == 0, "no strip fill without a shake")
+
+  love.graphics.draw, love.graphics.rectangle = origDraw, origRect
+  PaletteFX.shader, PaletteFX.sendColors = origShaderFn, origSend
+  PaletteFX.permute = origPermute
+end
 end
 
 -- ================= BUGS.md batch: ledge-shadow =================