Although ...

If the issue is "one frame behind" ...

Quakespasm source ...

scr_drawdialog = true;
SCR_UpdateScreen (); <---- I don't draw twice!
scr_drawdialog = false;
...
do {
..
while }

If the stream expects double buffering, it isn't getting it in SCR_ModalMessage.

The test for a solution would be doubling it up ...

scr_drawdialog = true;
SCR_UpdateScreen ();
SCR_UpdateScreen (); // Double buffer it.
scr_drawdialog = false;

(My first reply was on gut instinct about the event queue/not having a normal frame ... because I've experienced a fair number of headaches before due the blocking dialog so I had the blocking dialed in as public nuisance.

But gut instincts aren't always right ...

and although I've run into instances of the blocking dialog not drawing specifically because it is blocking .... may not mean anything at all in this situation ...) 

not all systems even support explicit *SwapBuffer calls.
Eg, webgl takes away control of your main loop, redrawning only when your redraw function returns.
There's other systems that do the same sort of thing, including Android's GLSurfaceView or MacOS's cocoa api.

Plus there are drivers that violate the GL spec and force triple-buffering, so you're never sure how many swaps you actually need to make to ensure that its actually swapped.

As a result, its imho easier to just keep redrawing the screen regardless (which is required on many systems anyway, where the system doesn't repaint unless the app explicitly does so - like windows).
But its best to avoid modal things entirely. Sometimes you don't really have a choice though (like QC debugging).

Regarding video capture, the recording program probably set up some code injection that copies the backbuffer to a pbo (or a compute shader) before the swap. Such things generally check the response the frame after that, which avoids forcing the cpu to sync with the gpu. This requires a couple more swaps before the data is available to encode, which of course makes loading screens really messy.
Hurrah for threads!... 

Am I allowed to sell a mod on steam, using quakespasm engine ? I need some guidance with GNU/GPLv2. 

Here is an example of someone selling something on Steam using a Quake engine: https://store.steampowered.com/app/793670/The_Wastes/

Here is a mod on Steam for Half-Life: https://store.steampowered.com/app/225840/Sven_Coop/

The engine source code license agreement does not have much to do with any of this. 

As long as I give credits to quakespasm creators, and if the mod doesn't include quake materials (models, textures, sounds...) it's ok ? 

You should read this: https://partner.steamgames.com/steamdirect

But then also ask the question: are there people who are going to pay for a Quake map/mod in 2018?

Probably not. 

It's not about the engine, but the vibe (Thirty Flights of Loving, DUSK...)

Thank you for the link. 

In the mapping help thread, you asked about WAD3 (Half Life 1 texture format).

FTE is the engine used in the first link I gave you (to the best of my knowledge) and it supports the Half Life 1 map format, which is like basically Quake map format except for the texture format is different which is why J.A.C.K. can easily do both Quake and Half-Life 1 maps.

I mention that because you referenced something with tons of color, and it is basically impossible in Quake because of the color count restrictions. 

I stick with Quakespasm. Quake palette haven't much "blue" colors, right. I haven't yet looked into customizing palette.lmp

Do ya know PixaTool ? 

Is there any way to smooth out jerky monster movement while they ride platforms or elevators? 

I tend to store a ton of savegames with crazy names. I really like how the "map" command has auto-complete on the mapname.

Might it be possible to do a similar autocomplete on the "load" command for the savegame names? 

just a brainfart, not terribly important of course 

Are there plans to add Spike's frame-rate/physics fix to QS any time soon?

I'm just curious as I've found that it has fixed my issue with intermittent stuttering on my 144Hz monitor. :) 

QS has been on the backburner for me, but when I get back to it that is something I want to add. 

It's kinda surprising that Spike's 'little' breakthrough hasn't caused a bit more excitement. I suppose that's to be expected in such a small community. :) 

that and that anyone who actually cared already had a choice of other engines that already had fixes for it. 

I implemented compute shader water warp (one pass) in vkQuake. Feel free to steal it, can easily be translated to a fragment shader with a triangle over the texture. 

I have troubles running "Eargasm" mod for Quakespasm. It seems the engine doesn't accept the new wav files, at least some of them.
For instance :

guncock.wav is played in the engine, but it seems to be filtered/emulated in 11025hz/8bit.

buzz1.wav isn't played at all (I've checked with the command "snd_show 1", the engine doesn't play the sound at all).

I tried x86 and x64 Quakespasm.
I tried to re-encode the files using different wav format settings.

Latest Quakespasm update : 20th Nov 2017.
Eargasm was released on 16th Dec 2017.
Would it be possible to make an update ?

https://www.moddb.com/mods/quake-eargasm/downloads/quake-eargasm 

says it all. 

Suggestion: Add option to disable auto weapon switch on pickup 

That's the realm of QuakeC, and not something that can be done in engine without the hideousest of hacklets. 

Not sure if this helps but all non-music sounds must be mono in Quakespasm. Maybe there are some stereo files in that pack designed for use with other less compatible Quake engines. 

That explains why some of my sounds from a few mods fail in game. 

As far as my tests this is the case with QS. I don't have a PC at work today so I can't confirm. But as I see in the DP docs stereo support is mentioned. "Stereo sound file support (useful for cd tracks)" I'm assuming that this was added, some mods were created with stereo files that break compatibility with Fitz variants. Which mods are messed up? I'd like to pursue this further and if you want I can mix the tracks down to mono for you.