Enlarging by dragging: Background not in accordance to OS theme

[tbrodbeck]

Issue Type: Bug

Steps to Reproduce:

1. Use OS day theme
2. Start VS Code
3. Switch to OS night theme
4. Enlarge VS Code window size by dragging

Problem:

When enlarging the window has a white (day theme) background

Expected:

When enlarging the window would have a dark (night theme) background

Visual demonstration:

My.Movie.mp4

System information:

VS Code version: Code 1.60.2 (7f6ab54, 2021-09-22T11:59:27.195Z)
OS version: Darwin x64 20.6.0
Restricted Mode: No

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz (12 x 2600)
GPU Status	2d_canvas: enabled gpu_compositing: enabled metal: disabled_off multiple_raster_threads: enabled_on oop_rasterization: enabled opengl: enabled_on rasterization: enabled skia_renderer: disabled_off_ok video_decode: enabled webgl: enabled webgl2: enabled
Load (avg)	3, 3, 4
Memory (System)	16.00GB (0.09GB free)
Process Argv	--disable-extensions --crash-reporter-id 66972b6f-696c-4473-810b-c0357cc17a00
Screen Reader	no
VM	0%

Extensions disabled

A/B Experiments

vsliv368:30146709
vsreu685:30147344
python383cf:30185419
pythonvspyt602:30300191
vspor879:30202332
vspor708:30202333
vspor363:30204092
vstes627:30244334
pythonvspyt639:30300192
pythontb:30283811
pythonptprofiler:30281270
vshan820:30294714
vstes263cf:30335440
pythondataviewer:30285071
pythonvsuse255:30340121
vscod805cf:30301675
pythonvspyt200:30340761
binariesv615:30325510
vsccppwtct:30364498
pythonvssor306:30344512
bridge0708:30335490
pygetstartedt2:30371810
dockerwalkthru:30370836
bridge0723:30353136
pythonrunftest32cf:30365367
pythonf5test824:30361777
javagetstartedt:30364666
pythonvspyt187cf:30365362
pydsgst2:30361792
vssid140:30363603
vssur157:30367808

[sbatten]

@jrieken I assigned you because I wonder if this has to do with the "splash screen" experience you set up. Feel free to assign back if not.

[jrieken]

Unrelated to splash screen, likely theme caching

[bpasero]

I know this used to work at one point but maybe broke. Relevant code seems to be:

vscode/src/vs/platform/theme/electron-main/themeMainService.ts

Lines 77 to 84 in ee1655a

	private updateBackgroundColor(windowId: number, splash: IPartsSplash): void {
	for (const window of BrowserWindow.getAllWindows()) {
	if (window.id === windowId) {
	window.setBackgroundColor(splash.colorInfo.background);
	break;
	}
	}
	}