Steps to Reproduce:
When I hover the buttons, they appear. But as you can see, the modal header has some dark black color.

Does this issue occur when all extensions are disabled?: Yes
@HRK44 in the 1.26 update we moved to Electron 2.0.x and this brings GTK3 dialogs. Are you maybe using a custom theme in Ubuntu that could have an impact?
@bpasero I haven't changed anything in my Ubuntu setup, just upgraded VS code (was working fine with previous version). What kind of information from my settings would be useful for you?
@HRK44 thanks, we should try to reproduce this with a bare Electron to report it to them. Can you:

Console output:
@HRK44 looks pretty normal to me. Can you:
dialog.showErrorBox("Hello", "World");@bpasero I put it inside the callback, it wasn't rendering anything outside (line 25).

@HRK44 interesting that one looks OK too. Lets try to show the same dialog as the one in code, can you try this dialog instead: dialog.showMessageBox(mainWindow, { title: 'VSCode', message: 'Some message', button: ["Move"] });
@bpasero I commented the rest of the code (same behavior), the button is not showing for some reason

@HRK44 try with this: dialog.showMessageBox(mainWindow, { title: 'VSCode', message: 'Some message', detail: "this is some details", buttons: ["Move", "Cancel"], type: "warning", cancelId: 0 }, function() {})
@bpasero looks fine

@HRK44 ok here is the exact code to reproduce the same dialog:
dialog.showMessageBox(mainWindow, {
buttons: ["Cancel", "_Move"],
cancelId: 0,
checkboxLabel: "Do not ask me again",
defaultId: 1,
message: "Are you sure you want to move 'vsc-extension-quickstart.md'?",
noLink: true,
title: "Code - OSS Dev",
type: "question"
}, function () { })
@bpasero dialog shows fine

@HRK44 wow, I am out of ideas on this one. Is there anything logged to VSCode console (run with code --verbose) when the dialog opens?
Does this happen with all dialogs (e.g. try to close a dirty file)?
Here is the console when I start code, that might help :
[16397:0823/132212.476507:WARNING:browser_main_loop.cc(275)] Gtk: Locale not supported by C library.
Using the fallback 'C' locale.
[16397:0823/132212.674543:WARNING:browser_main_loop.cc(275)] Gtk: Error loading image 'file:///usr/share/themes/Ambiance/gtk-3.0/assets/entry.png': Unable to load image-loading module: /usr/lib/x86_64-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.so: /snap/core/current/lib/x86_64-linux-gnu/libz.so.1: version ZLIB_1.2.9' not found (required by /usr/lib/x86_64-linux-gnu/libpng16.so.16)
[16397:0823/132212.675331:WARNING:browser_main_loop.cc(275)] Gtk: Error loading image 'file:///usr/share/themes/Ambiance/gtk-3.0/assets/button.png': Unable to load image-loading module: /usr/lib/x86_64-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.so: /snap/core/current/lib/x86_64-linux-gnu/libz.so.1: versionZLIB_1.2.9' not found (required by /usr/lib/x86_64-linux-gnu/libpng16.so.16)
[16397:0823/132212.675873:WARNING:browser_main_loop.cc(275)] Gtk: Error loading image 'file:///usr/share/themes/Ambiance/gtk-3.0/assets/button-toolbar.png': Unable to load image-loading module: /usr/lib/x86_64-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.so: /snap/core/current/lib/x86_64-linux-gnu/libz.so.1: version `ZLIB_1.2.9' not found (required by /usr/lib/x86_64-linux-gnu/libpng16.so.16)
libGL error:
unable to load driver: i965_dri.so
libGL error:
driver pointer missing
libGL error:
failed to load driver: i965
libGL error:
unable to load driver: i965_dri.so
libGL error:
driver pointer missing
libGL error:
failed to load driver: i965
libGL error:
unable to load driver: swrast_dri.so
libGL error:
failed to load driver: swrast
[16431:0823/132212.953380:ERROR:gl_context_glx.cc(186)] Failed to create GL context with glXCreateContextAttribsARB.
[16431:0823/132212.953448:ERROR:gpu_info_collector.cc(51)] gl::init::CreateGLContext failed
[16431:0823/132212.953454:ERROR:gpu_info_collector.cc(119)] Could not create context for info collection.
[16431:0823/132212.953465:ERROR:gpu_init.cc(96)] gpu::CollectGraphicsInfo failed (fatal).
[16431:0823/132212.972915:ERROR:gpu_child_thread.cc(252)] Exiting GPU process due to errors during initialization
[16397:0823/132212.978112:ERROR:browser_gpu_channel_host_factory.cc(103)] Failed to launch GPU process.
[main 1:22:13 PM] Sending env to running instance...
[main 1:22:13 PM] Sent env to running instance. Terminating...
[main 1:22:13 PM] Lifecycle#kill()

It happens with all dialogs.
More logs :

ESC[90m[main 1:50:46 PM]ESC[0m windowsService#showMessageBox 1
Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.
[19768:0823/135046.951999:WARNING:browser_main_loop.cc(275)] Gtk: Error loading image 'file:///usr/share/themes/Ambiance/gtk-3.0/assets/backdrop-check-unselected.png': Unable to load image-loading module: /usr/l
ib/x86_64-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.so: /snap/core/current/lib/x86_64-linux-gnu/libz.so.1: version ZLIB_1.2.9' not found (required by /usr/lib/x86_64-linux-gnu/libpng16.so.16)
[19768:0823/135046.952045:WARNING:browser_main_loop.cc(275)] Gtk: drawing failure for widget 'GtkCheckButton': invalid matrix (not invertible)
[19768:0823/135046.952054:WARNING:browser_main_loop.cc(275)] Gtk: drawing failure for widget 'GtkBox': invalid matrix (not invertible)
[19768:0823/135046.952060:WARNING:browser_main_loop.cc(275)] Gtk: drawing failure for widget 'GtkBox': invalid matrix (not invertible)
[19768:0823/135046.952065:WARNING:browser_main_loop.cc(275)] Gtk: drawing failure for widget 'GtkBox': invalid matrix (not invertible)
[19768:0823/135046.952071:WARNING:browser_main_loop.cc(275)] Gtk: drawing failure for widget 'GtkMessageDialog': invalid matrix (not invertible)
[19768:0823/135046.985482:WARNING:browser_main_loop.cc(275)] Gtk: Error loading image 'file:///usr/share/themes/Ambiance/gtk-3.0/assets/check-unselected.png': Unable to load image-loading module: /usr/lib/x86_64-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.so: /snap/core/current/lib/x86_64-linux-gnu/libz.so.1: versionZLIB_1.2.9' not found (required by /usr/lib/x86_64-linux-gnu/libpng16.so.16)
[19768:0823/135046.986894:WARNING:browser_main_loop.cc(275)] Gtk: drawing failure for widget 'GtkCheckButton': invalid matrix (not invertible)
[19768:0823/135046.987664:WARNING:browser_main_loop.cc(275)] Gtk: drawing failure for widget 'GtkBox': invalid matrix (not invertible)
[19768:0823/135046.987972:WARNING:browser_main_loop.cc(275)] Gtk: drawing failure for widget 'GtkBox': invalid matrix (not invertible)
[19768:0823/135046.987990:WARNING:browser_main_loop.cc(275)] Gtk: drawing failure for widget 'GtkBox': invalid matrix (not invertible)
[19768:0823/135046.987997:WARNING:browser_main_loop.cc(275)] Gtk: drawing failure for widget 'GtkMessageDialog': invalid matrix (not invertible)

[19768:0823/135133.139056:WARNING:browser_main_loop.cc(275)] Gtk: Error loading image 'file:///usr/share/themes/Ambiance/gtk-3.0/assets/button-default-focused-hover.png': Unable to load image-loading module: /usr/lib/x86_64-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.so: /snap/core/current/lib/x86_64-linux-gnu/libz.so.1: version ZLIB_1.2.9' not found (required by /usr/lib/x86_64-linux-gnu/libpng16.so.16)
[19768:0823/135133.450674:WARNING:browser_main_loop.cc(275)] Gtk: Error loading image 'file:///usr/share/themes/Ambiance/gtk-3.0/assets/button-hover.png': Unable to load image-loading module: /usr/lib/x86_64-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.so: /snap/core/current/lib/x86_64-linux-gnu/libz.so.1: versionZLIB_1.2.9' not found (required by /usr/lib/x86_64-linux-gnu/libpng16.so.16)
[19768:0823/135142.326246:INFO:CONSOLE(1397)] "%cTRACE", source: file:///snap/vscode/51/usr/share/code/resources/app/out/vs/workbench/workbench.main.js (1397)
[19768:0823/135142.349043:WARNING:x11_util.cc(1353)] X error received: serial 5458, error_code 3 (BadWindow (invalid Window parameter)), request_code 3, minor_code 0 (X_GetWindowAttributes)
[19768:0823/135142.355870:INFO:CONSOLE(1397)] "%cTRACE", source: file:///snap/vscode/51/usr/share/code/resources/app/out/vs/workbench/workbench.main.js (1397)
@HRK44 good one, I think this is because we are using a PNG icon for the dialog and that seems to break the dialog. Can we try that with the electron-fiddle again, this time using an icon:
mainWindow = new BrowserWindow({ width: 800, height: 600, icon: '/home/ticino/Desktop/code.png' })
and the icon being:

@bpasero it works fine with the png icon.
It might have something to do with this issue : https://forum.snapcraft.io/t/skype-exits-when-trying-to-add-a-file-to-a-chat/6138/8

After further searching, there is this issue, that might be on the same problem.
@HRK44 I believe that issue is rather due to the ArchLinux OS (see https://github.com/Microsoft/vscode/issues/55934).
Would you be able to fix the ZLIB_1.2.9 issue to see if that helps? Sounds like a missing dependency to me?
@bpasero I tried some stuff but couldn't manage to successfully update this ZLIB without creating a bunch of system issues, if someone with more advanced skills in Linux would try that'd be great.
I just re-installed VS Code via apt and removed it from snap (actually I completely removed snap...) and now it works correctly.
Basically the issue is that snap is not using the latest library available on the system (libz.so.1) thus messing up other libraries that depend on it (GTK3 on ubuntu 18).
@HRK44 thanks!
FYI @flexiondotorg
For reference, the issue is not due to an incorrect zlib version suggested above. The problem is due to the snap accidentally referencing system libraries instead of it's own. This manifests with vscode loading the libgdk-pixbuf-loader library from the system, which is compiled against a different zlib than the one within the snap's files. The solution, therefore, is not to mess about trying to change the zlib version but to determine why it is using the system libraries in the first place.
I'll try to look at it tomorrow, or @popey or @flexiondotorg might beat me to it.
Any chance on getting a status update on this issue? I am facing the same thing. It would be no great hardship to try HRK44's solution but if there's any hope of seeing this resolved in the near-term I'd just as soon stick with the snap.
Today, I got VSCode version 1.32.3 (via Snap!) and the bug seems to be fixed!
Whereas 1.32.3 is the first version I've noticed has this problem. I reverted to 1.31.1 (I'm using Snap, by the way) and the problem went away. I then refreshed back to 1.32.3 and the problem returned.
Name Version Rev Tracking Publisher Notes
core 16-2.37.4 6531 stable canonical✓ core
vscode 1.32.3-1552606978 89 stable snapcrafters classic
Thanks to @flexiondotorg, this is now fixed in master! https://github.com/Microsoft/vscode/pull/70884
Still broken having removed Snapcrafter's vscode snap and installed code 1.33. :-(
Most helpful comment
For reference, the issue is not due to an incorrect zlib version suggested above. The problem is due to the snap accidentally referencing system libraries instead of it's own. This manifests with vscode loading the libgdk-pixbuf-loader library from the system, which is compiled against a different zlib than the one within the snap's files. The solution, therefore, is not to mess about trying to change the zlib version but to determine why it is using the system libraries in the first place.
I'll try to look at it tomorrow, or @popey or @flexiondotorg might beat me to it.