Mouse scroll wheel stops working in Glass Masquerade (511470) when gamepad is emulating a mouse

Compatibility Report

• Name of the game with compatibility issues: Glass Masquerade
• Steam AppID of the game: 511470

System Information

• GPU: NVIDIA Corporation GeForce RTX 2080/PCIe/SSE2
• Driver/LLVM version: 4.6.0 NVIDIA 430.14
• Kernel version: 4.19
• Link to full system information report as Gist: https://gist.github.com/diegov/eef73c8c70469be4f0a0df3c23ea56fd
• Proton version: 4.2-7

I confirm:

• that I haven't found an existing compatibility report for this game.
• that I have checked whether there are updates for my system available.

Symptoms

When launching the game from Big Picture, using a gamepad configured to emulate a mouse, the mouse's scroll wheel doesn't work. Also the ESC key doesn't work (possibly other keyboard keys, the game I think only responds to ESC.) If I then switch to the physical mouse, its scroll wheel doesn't work either.

Strangely, if I first move around the game with a physical mouse, and then switch to the gamepad, then it all works fine, both the physical mouse and the emulated one, including the scroll wheel.

Reproduction

In Big Picture mode:

• Configure a bluetooth connected Dualshock 4 gamepad for the game using the template for WASD keyboard and mouse
• Launch the game
• Without moving the physical mouse or using the physical keyboard, start a level
• Hover over one of the puzzle pieces that have yet to be placed
• Press L1 or R1 (which map to the mouse's scroll wheel)

Expected Result

The circle where the pieces are placed rotates to show more pieces

Actual Result

The circle doesn't move

Notes

I can confirm the scroll wheel and ESC key work fine no matter what the first input is in proton 3.16-9 beta, and I'm pretty sure it was all working fine on previous proton 4.2 versions (4.2.6 or maybe 4.2.5) though I can't confirm it now.

Reproduce the issue in Proton 4.2-7 using Big Picture, a Bluetooth DualShock 4 configured for WASD and mouse, and Glass Masquerade (AppID 511470). Start by tracing the input handling involved when the gamepad provides the first input; the work is done when scroll-wheel input and ESC work whether input begins with the gamepad or physical devices.