Request: ability to set scale factor without recreating a layout

[nicoburns]

It should be possible to set the scale factor and relayout without completely recreating the layout, reshaping, etc.

Use case: relayouting when moving a window from one monitor to another where those monitors have different scale factors.

[DJMcNab]

One thing to think about here is the floating point related implications; if you change the scale factor multiple times, will you always end up at the same values? This would be no matter the chain of (reasonable) scale factors you use (i.e. $$2$$, $$\frac{3}{2}$$, $$\frac{4}{3}$$, $$\frac{9}{4}$$ - which is used on Android).

I think the scale differences are likely to be small enough that it's imperceptible, but this would I think be the main technical barrier here.

[nicoburns]

One thing to think about here is the floating point related implications; if you change the scale factor multiple times, will you always end up at the same values?

I don't think this will be a problem because (at least my intention) is not to "fixup" the computed layout metrics, we would fully recompute the actual layout (size and placement of glyphs, etc) but just skip the computation stages (building a Layout, shaping and font-fallback) that don't actually use the scale at all.