The scale change from magnified focus to compressed context has to stay continuous, not jump abruptly

What it is

The boundary between focus and context is where this pattern succeeds or fails: the scale change from magnified focus to compressed context must be continuous and legible — graduated magnification, deliberate boundary treatment, a traceable correspondence. A hard cliff (half normal, half squashed; a sudden seam) breaks the sense of correspondence at exactly the line users cross most, and "where does the focus end" becomes a permanent small confusion.

Why it happens

A legible transition sustains "correspondence tracking": elements seen in the focus region must map seamlessly onto the same elements in the context, which requires two things — a continuous scale change (adjacent elements differ in size by less than a perceptible jump) and a visible mapping (the user can tell which stretch of the context the focus magnifies). Scale cliffs destroy the former: adjacent elements across the boundary jump in size, severing the perception that they belong together; poor boundary treatment destroys the latter: the focus reads as a pane of glass floating above the context with no registration to it. Graduated transitions (fisheye's continuous warp) satisfy continuity naturally; block-style designs (collapsible trees) must compensate with explicit correspondence markers (expansion arrows, synced highlights).

Where it stops holding

Continuity need not be geometric: a collapsed tree has a hard boundary at each expand point, yet the arrow and the indent structure supply clear structural correspondence — legibility achieved. Conversely, geometric continuity with semantic breakage (warping that overlaps or displaces elements) does harm. Magnitude needs restraint too — the higher the focus magnification, the fiercer the peripheral compression and the harder the boundary reads; high-magnification demand should switch to the dual-view arrangement rather than stretch the warp. Animated transitions (continuous deformation while the focus moves) aid correspondence enormously, subject to the reduced-motion conventions.

Applying it

• Cap the size step between elements across the focus boundary; beyond the cap, smooth the transition or add correspondence markers.
• For block-style focus (collapse, drawers), provide structural indicators at the boundary (arrows, indentation, numbering).
• Verification: have users pick an element in the focus region and point to its counterpart in the context; fluent pointing means the transition passes — repeated hunting means correspondence is broken; add transition or markers.

Related

• Same group: U6.06.1 Focus plus context keeps detail and global in one view · U6.06.2 The difference from overview-plus-detail is whether they occupy two views · U6.06.4 The pattern saves gaze switches but adds spatial comprehension load · U6.06.5 The focus position must be movable by the user
• Nearby: U6.07.2 Geometric distortion breaks positional encoding; magnitudes stop being readable · U11.03.4 Transition animations must respect the system's reduced-motion preference
• Search terms: focus context transition · degree of interest · distortion boundary