Splitting a custom editor multiple times results in too many editor instances

[mjbvz]

Issue Type: Bug

repro

1. On mac.
2. Open the built-in image preview
3. Hold down cmd then press \ to split
4. Continue holding down cmd but now press \ again

Expected
There should be 3 editors open.

Bug
There are now 4 editors open.

VS Code version: Code - Insiders 1.45.0-insider (6e9a34c, 2020-04-16T05:41:45.257Z)
OS version: Darwin x64 19.4.0

System Info

Item	Value
CPUs	Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz (16 x 2400)
GPU Status	2d_canvas: enabled flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled metal: disabled_off multiple_raster_threads: enabled_on oop_rasterization: disabled_off protected_video_decode: unavailable_off rasterization: enabled skia_renderer: disabled_off_ok video_decode: enabled viz_display_compositor: enabled_on viz_hit_test_surface_layer: disabled_off_ok webgl: enabled webgl2: enabled
Load (avg)	3, 3, 3
Memory (System)	32.00GB (1.17GB free)
Process Argv	-psn_0_12385231
Screen Reader	no
VM	17%