Chat from 'fork' conversation went to master and modified one file in two threads

Type: <b>Bug</b>

Main conversation was processing.
I did a fork from previous answer and added some comment - copilot started to work on it in a fork.
Then when I switched to main and I saw that it responded to comment from the fork and then it started to modify the same files in two threads!

Extension version: 2.1.117
VS Code version: Code 1.116.0 (Universal) (560a9dba96f961efea7b1612916f89e5d5d4d679, 2026-04-15T00:28:13Z)
OS version: Darwin arm64 25.2.0
Modes:

<details>
<summary>System Info</summary>

|Item|Value|
|---|---|
|CPUs|Apple M4 Pro (12 x 2400)|
|GPU Status|2d_canvas: enabled<br>GPU0: VENDOR= 0x106b [Google Inc. (Apple)], DEVICE=0x0000 [ANGLE (Apple, ANGLE Metal Renderer: Apple M4 Pro, Version 26.2 (Build 25C56))], DRIVER_VENDOR=Apple, DRIVER_VERSION=26.2 ACTIVE<br>Machine model name: Mac<br>Machine model version: 16.8<br>direct_rendering_display_compositor: disabled_off_ok<br>gpu_compositing: enabled<br>multiple_raster_threads: enabled_on<br>opengl: enabled_on<br>rasterization: enabled<br>raw_draw: disabled_off_ok<br>skia_graphite: enabled_on<br>trees_in_viz: disabled_off<br>video_decode: enabled<br>video_encode: enabled<br>webgl: enabled<br>webgl2: enabled<br>webgpu: enabled<br>webnn: disabled_off|
|Load (avg)|2, 3, 3|
|Memory (System)|24.00GB (0.07GB free)|
|Process Argv|--crash-reporter-id c055cd1a-e76a-4eda-b52e-d43201b43264|
|Screen Reader|no|
|VM|0%|

</details><details>
<summary>A/B Experiments</summary>

vsliv368cf:30146710
binariesv615:30325510
nativeloc1:31344060
dwcopilot:31170013
dwoutputs:31242946
copilot_t_ci:31333650
e5gg6876:31282496
pythonrdcb7:31342333
6518g693:31463988
aj953862:31281341
63221493:31336333
envsactivate1:31464700
cloudbuttont:31379625
3efgi100_wstrepl:31403338
use-responses-api:31390855
ddidtcf:31399634
je187915:31401257
ec5jj548:31422691
cp_cls_c_966_ss:31454199
c3h7c220:31478652
ge8j1254_inline_auto_hint_haiku:31490510
38bie571_auto:31478677
cp_cls_t_1081:31454832
conptydll_true:31489736
ia-use-proxy-models-svc:31452481
core-renderer-profiling-on:31440367
e9c30283:31461165
test_treatment2:31471001
control_6dc23131:31495068
idci7584:31464702
e3e4d672:31494082
ei9d7968:31496641
534a6447:31496642
nes-extended-on:31455476
chat:31457767
8hig5102:31480529
67jbj424:31495046
cpptoolson-v2:31475363
i2gc6536:31494041
1g3cj959:31494757
inline_hover_fd9bg283:31492073
23c7c724:31491644
client_tst_t:31495907
po_split_t:31488386
ddid_c:31478207
getcmakediagnosticson:31489824
ja75b849:31495667
nes-perm-reb-0:31490411
cp_cls_c_1082:31491634
ihfe3146:31493005
719di409_sum_t:31495624
0he74425_c:31489329
logging_disabled_new:31493366
output_2x:31496635

</details>

Main conversation:
<!-- generated by issue reporter -->
<img width="1100" height="942" alt="Image" src="https://github.com/user-attachments/assets/4020488a-2707-45c4-a928-b39609cc70db" />
Fork with comment about the formatting:
<img width="1073" height="425" alt="Image" src="https://github.com/user-attachments/assets/b37bb53e-da2f-4476-b603-cb3fa0e85c3a" />