Markdown preview stops Ctrl+` from toggling terminal

[octref]

Issue Type: Bug

Repro:

• Open Markown preview
• Ctrl + ` for a few times

VS Code version: Code - Insiders 1.31.0-insider (5ab047b, 2019-02-04T15:24:24.455Z)
OS version: Darwin x64 17.7.0

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-4870HQ CPU @ 2.50GHz (8 x 2500)
GPU Status	2d_canvas: enabled checker_imaging: disabled_off flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on native_gpu_memory_buffers: enabled rasterization: enabled surface_synchronization: enabled_on video_decode: enabled webgl: enabled webgl2: enabled
Load (avg)	3, 2, 2
Memory (System)	16.00GB (2.05GB free)
Process Argv	-psn_0_1364301
Screen Reader	no
VM	0%

[Tyriar]

I can repro, the panel flickers for me sometimes. I don't think this meets the bar for 1.31 though as there's many workarounds (run command, focus an editor, etc.) and only happens sometimes.