diff --git a/tests/test-client-borders.lua b/tests/test-client-borders.lua
new file mode 100644
index 000000000..d9d044e7c
--- /dev/null
+++ b/tests/test-client-borders.lua
@@ -0,0 +1,190 @@
+-- Brute force every possible states and see what blows up.
+--
+-- The goal is to detect "other" code paths within the core
+-- which have the side effect of accidentally marking the border
+-- to be user-specified. When this happens, all theme variables
+-- stop working.
+--
+-- For example, fullscreen caused an issue because the default
+-- request::geometry modified the border_width.
+local beautiful = require("beautiful")
+local aclient = require("awful.client")
+local test_client = require("_client")
+
+local steps = {}
+local used_colors = {}
+local state_colors, state_widths = {}, {}
+local state_setter = {}
+local c = nil
+
+-- Make sure it uses `beautiful.border_width` (default) again.
+local function reset()
+    client.focus = nil
+    c.urgent     = false
+    c.maximized  = false
+    c.fullscreen = false
+
+    -- Use the low level API because `floating` has an implicit/explicit
+    -- mode just like the border.
+    aclient.property.set(c, "floating", nil)
+end
+
+local function gen_random_color(state)
+    while true do
+        local str = "#"
+        for _=1, 3 do
+            local part = string.format("%x", math.ceil(math.random() * 255))
+            part = #part == 1 and ("0"..part) or part
+            str = str .. part
+        end
+
+        if not used_colors[str] then
+            used_colors[str] = state
+            return str
+        end
+    end
+end
+
+local function check_state(state)
+    reset()
+
+    if state_setter[state] then
+        local col = "border_color" .. state
+        local w = "border_width" .. state
+        state_setter[state](client.get()[1])
+
+        assert(c.border_color ~= nil)
+        assert(c.border_width ~= nil)
+        assert(
+            c.border_color == state_colors[col],
+            "Expected "..state_colors[col].." for "..state.. " but got "..c.border_color..
+            " for "..(used_colors[c.border_color] or "nil")
+        )
+
+        if not state:find("full") then
+            assert(c.border_width == state_widths[w])
+        else
+            assert(c.border_width == 0)
+        end
+
+        assert(not c._private._user_border_width)
+        assert(not c._private._user_border_color)
+    end
+
+    return true
+end
+
+local function clear_theme()
+    for k in pairs(beautiful) do
+        if k:find("border_") then
+            beautiful[k] = nil
+        end
+    end
+
+    return true
+end
+
+for _, state1 in ipairs {"_fullscreen", "_maximized", "_floating" } do
+    local color = "border_color" .. state1
+    local width = "border_width" .. state1
+
+    state_widths[width] = math.floor(math.random() * 10)
+    state_colors[color] = gen_random_color(color)
+
+    for _, state2 in ipairs {"_urgent", "_active", "_new", "_normal" } do
+        color = "border_color" .. state1 .. state2
+        width = "border_width" .. state1 .. state2
+
+        state_widths[width] = math.floor(math.random() * 10)
+        state_colors[color] = gen_random_color(color)
+    end
+end
+
+for _, state in ipairs {"_urgent", "_active", "_new", "_normal" } do
+    local color = "border_color" .. state
+    local width = "border_width" .. state
+
+    state_widths[width] = math.floor(math.random() * 10)
+    state_colors[color] = gen_random_color(color)
+    beautiful[width] = state_widths[width]
+    beautiful[color] = state_colors[color]
+end
+
+function state_setter._urgent()
+    c.urgent = true
+end
+
+function state_setter._fullscreen()
+    c.fullscreen = true
+end
+
+function state_setter._floating()
+    c.floating = true
+end
+
+function state_setter._active()
+    client.focus = c
+end
+
+function state_setter._maximized()
+    c.maximized = true
+end
+
+function state_setter._normal()
+    -- no-op
+end
+
+test_client()
+
+table.insert(steps, function()
+    c = client.get()[1]
+    return c and true or nil
+end)
+
+for _, state in ipairs {"_urgent", "_active", "_new", "_normal" } do
+    table.insert(steps, function()
+        return check_state(state)
+    end)
+end
+
+table.insert(steps, clear_theme)
+
+table.insert(steps, function()
+    for _, state in ipairs {"_fullscreen", "_maximized", "_floating" } do
+        local color = "border_color" .. state
+        local width = "border_width" .. state
+
+        beautiful[width] = state_widths[width]
+        beautiful[color] = state_colors[color]
+    end
+
+    return true
+end)
+
+for _, state in ipairs {"_fullscreen", "_maximized", "_floating" } do
+    table.insert(steps, function()
+        return check_state(state)
+    end)
+end
+
+table.insert(steps, clear_theme)
+
+table.insert(steps, function()
+    -- Add everything to the theme.
+    for _, mode in ipairs {state_colors, state_widths} do
+        for key, value in pairs(mode) do
+            beautiful[key] = value
+        end
+    end
+    return true
+end)
+
+for _, state1 in ipairs {"_fullscreen", "_maximized", "_floating" } do
+    for _, state2 in ipairs {"_urgent", "_active", "_new", "_normal" } do
+        table.insert(steps, function()
+            return check_state(state1 .. state2)
+        end)
+    end
+end
+
+require("_runner").run_steps(steps)