VST3: Fix DPI scaling issues caused by JUCE 8.0.13 DPI changes

[emezeske]

Fix double-scaled plug-in editors on Windows in hosts that send a redundant content scale

The problem

On a high-DPI Windows display the plug-in UI has to be drawn bigger (e.g. 1.25x at 125%). There are two independent ways a plug-in can be told to do that:

1. The OS way: the window is marked per-monitor DPI-aware, and Windows scales it.
2. The host way: the host calls IPlugViewContentScaleSupport::setContentScaleFactor(...).

Some hosts do both at once for the same factor: the window is DPI-aware and the host also sends a content scale equal to the monitor's DPI. When that happens, if the desktop scaling is e.g. 1.25 JUCE applies the 1.25 twice and the editor renders at ~1.56x.

I observed this in Renoise, where the plugin window is too large and scaled up too much.

Why it works in most hosts but breaks in Renoise

Most hosts pick one mechanism: per-monitor-aware hosts (Reaper, Cubase, etc.) rely on the OS and send no content scale (or 1.0). Renoise (and Tracktion Waveform) are DPI-aware and call setContentScaleFactor with a factor equal to the monitor DPI, so the same scale is counted twice. It is not a Renoise-specific bug; it is any host that supplies a content scale a DPI-aware window does not need.

Why old JUCE worked and 8.0.13 broke it

This regressed in fcf1971122 ("Plugin Client: Change scaling mechanism on Linux/Windows plugins"), which changed how the host content scale is applied:

• Before 8.0.13: the host content scale was applied as a top-level AffineTransform on the editor, independent of the window's OS/peer DPI scaling. The two lived in separate channels and did not stack, so a redundant host scale was harmless.
• In 8.0.13: the host content scale is applied as the window's platform scale (ComponentPeer::setCustomPlatformScaleFactor), and convertToHostBounds now multiplies by desktopScale * platformScale. The host scale and the window's own DPI scaling now occupy the same channel, so a host scale that equals the DPI gets multiplied in on top of it, leading to double-scaling.

The fix

In JuceVST3Editor::setContentScaleFactor, ignore any content scale that is at or below the window's own DPI scale (getScaleFactorForWindow). The OS already applies that much to a DPI-aware window, so such a factor carries no new information; only a factor that exceeds the window's DPI represents extra scaling the host actually wants.

This is correct in every case:

• DPI-aware window, redundant scale (Renoise / Waveform): ignored: the OS's single DPI scaling stands.
• DPI-unaware window: getScaleFactorForWindow returns 1.0, so a real host scale (> 1.0) is never mistaken for redundant and is still applied.
• Cubase 10 integer-scale workaround (the existing block right below): its integer factor never equals the fractional window DPI, so that path is untouched.
• Genuine host zoom beyond the monitor DPI: a factor greater than the window scale is still honored.

It restores the pre-8.0.13 result (a single DPI scaling) without reverting the new mechanism, and is a no-op for any host that was not already sending a redundant content scale.

[juce-push-bot]

This pull request has been mentioned on The JUCE Forum. There might be relevant details there:

https://forum.juce.com/t/windows-dpi-scaling-broken-in-juce-8-0-13-for-some-daws-renoise/68923/1

[emezeske]

I have manually tested this change in recent versions of the following DAWs:

Ableton (with auto-scale both on and off)
Bitwig
Cakewalk
Cubase
FL Studio
Gig Performer
Pro Tools
Reaper
Reason
Renoise
Soundminer
Studio One
Waveform

[juce-push-bot]

This pull request has been mentioned on The JUCE Forum. There might be relevant details there:

https://forum.juce.com/t/juce-8-0-13-released/68851/24