terminal: report DECECM as permanently reset

src/terminal/modes.zig

@@ -79,6 +79,11 @@ pub const ModeState = struct {
     /// Return a DECRPM report for the given mode tag. If the tag does
     /// not correspond to a known mode, the report state is .not_recognized.
     pub fn getReport(self: *const ModeState, tag: ModeTag) Report {
+        // DECECM (Erase Color Mode) is DEC private mode 117. Ghostty behaves as
+        // if it is permanently reset and does not implement it as a mutable mode.
+        if (!tag.ansi and tag.value == 117) {
+            return .{ .tag = tag, .state = .permanently_reset };
+        }
         const mode = modeFromInt(tag.value, tag.ansi) orelse return .{
             .tag = tag,
             .state = .not_recognized,
@@ -347,6 +352,13 @@ test "getReport known ANSI mode" {
     try testing.expectEqual(true, report.tag.ansi);
 }
 
+test "getReport DECECM permanently reset" {
+    const state: ModeState = .{};
+    const report = state.getReport(.{ .value = 117, .ansi = false });
+    try testing.expectEqual(Report.State.permanently_reset, report.state);
+    try testing.expectEqual(false, report.tag.ansi);
+}
+
 test "getReport unknown mode" {
     const state: ModeState = .{};
     const report = state.getReport(.{ .value = 9999 });

src/terminal/stream_terminal.zig

@@ -1366,6 +1366,10 @@ test "request mode DECRQM with write_pty callback" {
         // Query an unknown mode
         s.nextSlice("\x1B[?9999$p");
         try testing.expectEqualStrings("\x1B[?9999;0$y", S.last_response.?);
+
+        // Query DECECM, which Ghostty recognizes but does not allow changing
+        s.nextSlice("\x1B[?117$p");
+        try testing.expectEqualStrings("\x1B[?117;4$y", S.last_response.?);
     }
 }