Why is there a black lag every time a WPF window is resized?

Question

Other questions on SE address how to speed up nested UI control resizing, but- what if there aren't any controls?

As you drag the edge of a WPF window, even a main window with no content, black bars flicker briefly during the drag. This produces a crummy feel- one that I don't want to inflict on customers:

It does get slower and heavier with a full UI on top of it as well. This doesn't even get into how ugly it looks when resizing using the top or left edges. Windows Forms- even with the heaviest UI I've built- never looks this bad right off the bat.

What can be done to make WPF window resizing performance comparable to win forms?

(I have Windows 7 x64 and a triple monitor system on an AT Radeon HD 7470.)

Answer 1

You could update your graphic card and try it out again but that wont change anything. The reason is pretty simple. We all get to see this sometimes based how fast/slow our computer is. Sometimes it runs smooth because we do not have many visuals to draw. The reason is no proper background color is found in graphic card at that moment in redrawing process. Your drivers are fine, and its not just because you use Wpf. Other techniques use the same mechanism behind redrawing.

The first thing WPF will do is clear out the dirty region that is going to redraw. The purpose of dirty regions is to reduce the amount of pixels sent to the output merger stage of the GPU pipeline. Here is where we see the black color. Window itself at that point has no background color or its background color is set to transparent and so to us the GPU draws the black background. Things run async in wpf which is good so.

To fix this you could set a fix color such as "White" to the Window. Then the WPF system will clean out the dirty region but fill it automatically with white color instead of black. This usually helps.

Match the window color or the color of top most layer. Dont let GPU use black and you should do fine. Btw Wpf is faster than WinForm so dont worry.

Answer 2

The look is crummy indeed, especially when using the top or left border.

Which exact problem your screen shot is showing depends on how long your app is taking to render as well as a couple of background related settings that you might be able to tweak to get better resize. Plus part of the ugly resize is specific to Aero.

While I can't address the specific crazy slowness of WPF redraw, I can at least give some insight on why you see black, where that is coming from, and whether you can change to a less annoying fill-in color.

It turns out there are multiple different sources of the black and the bad resize behavior from different Windows versions that combine together. Please see this Q&A which explains what is going on and provides advice for what to do (again, not specific to making WPF faster but just seeing what you can do given the speed you have):

How to smooth ugly jitter/flicker/jumping when resizing windows, especially dragging left/top border (Win 7-10; bg, bitblt and DWM)?