The red box around the current line in the tracer is not shown until you hit ED or TC in the tracer

[dyaandys]

  ⎕fx'foo' '1' '2' '3'
  foo TC

no red box around foo[1]
ER
red box around foo[2]

May be mistaken, but I think this used to work ..

IDE:
Version: 4.0.2853
Platform: Win32
Date: 2017-07-27 10:42:05 +0100
Git commit: e3f8266
Preferences:{
"favs":"[{"type":"connect","ssh_auth_type":"pass","ssh_tnl_auth_type":"pass","name":"debian7dev","host":"debian7dev.dyalog.bramley","port":"4502"},{"type":"connect","ssh_auth_type":"pass","ssh_tnl_auth_type":"pass","name":"dfsaix","host":"10.0.61.100","port":"9003"},{"ssh_auth_type":"pass","ssh_tnl_auth_type":"pass","name":"mac0","host":"mac0"},{"ssh_auth_type":"pass","ssh_tnl_auth_type":"pass","name":"local","host":"127.0.0.1","port":"9999","type":"start","exe":"C:\\Program Files\\Dyalog\\Dyalog APL-64 16.0 Unicode\\dyalog.exe"},{"ssh_auth_type":"pass","ssh_tnl_auth_type":"pass","name":"p7-61qa","host":"p7-61qa","port":"4503"}]",
"indent":"0",
"kbdLocale":"en_GB",
"lbarOrder":"← +-×÷*⍟⌹○!? |⌈⌊⊥⊤⊣⊢ =≠≤<>≥≡≢ ∨∧⍲⍱ ↑↓⊂⊃⊆⌷⍋⍒ ⍳⍸∊⍷∪∩~ /\⌿⍀ ,⍪⍴⌽⊖⍉ ¨⍨⍣.∘⍤@ ⍞⎕⍠⌸⌺⌶⍎⍕ ⋄⍝→⍵⍺∇& ¯⍬",
"lineNums":"0",
"pfkeys":"["","","{⍺ ⍵} ","FOO ","","","","","","","","",""]",
"selectedExe":"C:\Program Files\Dyalog\Dyalog APL-64 16.0 Unicode\dyalog.exe",
"title":"{WSID} {PID}",
"wse":"1",
"zoom":"-2"
}

Interpreter:
Version: 16.0.30448
Platform: Windows-64
Edition: Unicode/64
Date: Jul 21 2017 at 20:18:55

[flouc001]

@dyaandys please could you test the branch I have created and confirm the issue is fixed. Then I can merge into master and ride4. Thank you.

[dyaandys]

Looks fine to me.