Transition from one mask to another exposes sensitive information

[mstublefield]

Description

When having two masks adjacent to each other, especially when there is also a Zoom, the mask is dropped during the transition between the two masks, so the sensitive data becomes visible.

Reproduction

Have multiple masks adjacent to each other and a zoom. The intent was that as you zoomed in and out, the mask needed to move a little bit, but when you do this, the sensitive data becomes visible during the mask transition.

Additional Context

• Cap version: 0.4.71
• Operating system, version: macOS Tahoe 26.3
• Device (optional): Mac Studio

[linear]

CAP-646

[tembo]

Thanks for the detailed report. I looked into the codebase and can confirm this is a real bug in how adjacent mask segments are handled.

The issue is in crates/rendering/src/mask.rs:74:

if frame_time < segment.start || frame_time > segment.end {
    continue;
}

When two masks are defined adjacently (e.g., mask1.end = 5.0 and mask2.start = 5.0), the fade_duration logic causes a gap in coverage:

• Each mask has a default fade_duration of 0.15 seconds (configuration.rs:693)
• At the boundary frame, both masks are technically active but their intensity is multiplied by fade_in and fade_out values
• Since time_until_end = 0.0 for mask1 and time_since_start = 0.0 for mask2 at the exact boundary, both have intensity = 0 (multiplied by 0)
• During the fade transition window (0.15s before mask1 ends and 0.15s after mask2 starts), the combined intensity drops significantly

This creates a brief window where sensitive data becomes visible during the crossfade between masks.

For a workaround, overlapping mask segments by at least 2 * fade_duration (0.3 seconds by default) should maintain coverage. For example, if mask1 ends at 5.0s, have mask2 start at 4.7s.

A proper fix might involve:

1. Not applying fade-out when the next mask segment starts immediately (detecting adjacent segments)
2. Ensuring combined mask intensity never drops below a threshold when segments are adjacent
3. Computing the combined opacity of overlapping masks before applying to prevent coverage gaps