large images are not scaled to stay inside diff panel

Description

With the new image diff improvements in #2383 there’s a scenario where a large image is drawn outside the diff panel. We shouldn’t do this.

Version

GitHub Desktop version: current master e.g. 82b1d2377b98208d74aff0337c8f482cef2c1e35 OS version: Windows 10

Steps to Reproduce

1. Clone https://github.com/shiftkey/image-examples
2. Switch to History tab
3. Select this commit https://github.com/shiftkey/image-examples/commit/10c1cf4d08b884f83332d8ae24e14336c5ad73df
4. Ensure you have 2-up selected (but the others show this problem too)

Expected behavior:

• Image does not display outside diff view.
• Can toggle between different modes.

Actual behavior:

• Image drawn over top menu.
• Unable to change diff mode.

Reproduces how often: 100%

[shiftkey]

I’ve been able to reproduce this on Windows 10 and I think it’s some combination of scaling and screen real-estate.

This is the commit I’m working with (thanks @AndyThePie for the setup): https://github.com/AndyThePie/pizzaRPG-repo/commit/0233f1c56771ddf2d72f71c800d3f2ac86ef18ed

At 100% scaling on a 1440x900px display, the image diff is scaled down to fit into the space:

At 200% scaling on a 2560x1440px display, the image is no longer scaled down:

Probably requires some refreshing of what the image diff controls are rendering and whether they care about the available space…

[ChiriVulpes]

I use 125% zoom and I have this issue very frequently in an image-heavy repository. It can cover the branch/fetch buttons in the changes view, and always covers the diff view options in the history view. I have to resize down to 100% to work with this repository.