[Multi-Monitor] SWT applications to support and work with different zoom values per Shell.

[niraj-modi]

Copying @SyntevoAlex comment from #62 (comment)

To my understanding, the biggest blocker in current design is that in Windows, DPI is assigned per top-level window (Shell in terms of SWT) , whereas methods in DPIUtil do not use Shell as argument, so it uses a single DPI value for all monitors. With such design, we can only resort to supporting some scenarios at cost of other scenarios.

A proper fix would be to teach DpiUtil to consider Shell in question. Unfortunately, that will require massive (but probably not too complex) changes.

If someone volunteers to do that, I could assist to a reasonable extent.

[akoch-yatta]

Hello,

I am currently working on the HiDPI issues on Windows and got most of it to a decent state. The issue I am currently struggling with is how to adjust Fonts properly, when there are different zoom levels. I think for the system fonts (font == null) I found a proper way to handle it, but when an explicit font is set, I do not see a good solution. The main problem is how the make sure, that the font is applied correctly to each control depending on the monitor zoom. Having it refreshed when the zoom change is happening is fine, but when it is set in between, e.g. as by the CSS SWT Engine, that does not work properly.

1. The obvious idea was to adapt the place, where FontData is used to create a font. In Font.init(...) device.computePixels is called, which uses LOGPIXELSY to calculate the scale factor.
   int pixels = -(int)(0.5f + (height * OS.GetDeviceCaps(hDC, OS.LOGPIXELSY) / 72f));
   As this uses the primary monitor, this won't work in a monitor setup with different zoom levels. As there is only one instance of Display even with multiple monitors I don't see a way to handle it there.

2. Second idea was to adapt Control::setFont to resize fonts if necessary. That does not sound great as well. When called, there is already a font created and should be set for this control and this could lead to the creation of a new one that needs to be managed. Additionally one would need to recalculate the font size taking the zoom of the primary monitor and the one of the controls monitor into consideration, which sounds rather complicated.

Anyone has another idea for an approach about that topic or am I missing a basic concept here?