Canvas Initial Render & Continuous Repaint Fix

Problem Analysis

Issue 1: Canvas not visible on first image open

When a file is opened via pending_load in HcieApp::logic() (line 2897), the document is pushed to the dock using self.dock_state.push_to_focused_leaf(). This pushes the tab to whatever leaf is currently focused — which may be a utility panel (Tools, Layers, etc.) rather than the document center area. The document tab exists but is hidden behind a narrow utility panel, requiring the user to click the tab title to see the canvas.

The FileOpenRequested event handler (line 1573, 1604) correctly uses push_document_to_center() which finds the document leaf and focuses it before pushing. But the pending_load path in logic() uses the raw push_to_focused_leaf().

Issue 2: Continuous repaint

In logic() (line 2996-2998):

if self.stable_frames < 5 {
    ctx.request_repaint();
}

stable_frames is incremented in ui_main_assembly() (line 2370) only when (self.last_window_width - avail_w).abs() < 0.1. The layout_solver::adjust_pixel_widths() changes dock split fractions on startup (when stable_frames < 3), which can cause the available width to fluctuate slightly between frames, resetting stable_frames to 0 repeatedly. This creates an infinite loop of repaint requests.

Additionally, the enforce_constraints function runs every frame and may move document tabs around, which also triggers repaints.

Changes

1. `hcie-egui-app/crates/hcie-gui-egui/src/app/mod.rs` — `logic()` function

a) Use push_document_to_center() in pending_load handler

Replace both self.dock_state.push_to_focused_leaf(...) calls in the pending_load block (lines 2918 and 2944) with self.push_document_to_center(dock::HciePane::Document(self.active_doc)).

This ensures the document tab goes to the center document area and is focused immediately.

b) Fix continuous repaint — replace stable_frames < 5 with targeted approach

Replace the stable_frames < 5 check (lines 2995-2998) with a single ctx.request_repaint() call that runs only on the very first frame after app start. Use a new boolean field first_frame on HcieApp that is set to true in new() and set to false after the first repaint request.

This prevents the layout solver's width fluctuations from causing infinite repaint loops.

c) Ensure composite texture is created immediately after loading

After the pending_load block, explicitly call doc.engine.mark_composite_dirty() and ensure the composite buffer is allocated. This is already done by open_image() which sets composite_dirty = true, but we should also ensure the buffer is pre-allocated to the correct size so render_composition() can create the texture on the very first frame.

2. `hcie-egui-app/crates/hcie-gui-egui/src/app/mod.rs` — `HcieApp` struct

Add first_frame: bool field, initialized to true in new().

3. `hcie-egui-app/crates/hcie-gui-egui/src/app/mod.rs` — `ui_main_assembly()`

Ensure that after enforce_constraints moves document tabs, the composite is marked dirty so the canvas re-renders with the correct content.

Files to modify

File	Change
`hcie-egui-app/crates/hcie-gui-egui/src/app/mod.rs`	Fix `pending_load` to use `push_document_to_center()`, fix continuous repaint, add `first_frame` field

Files NOT to modify (regression protection)

hcie-engine-api/src/lib.rs — locked, no changes needed
hcie-egui-app/crates/hcie-gui-egui/src/canvas/mod.rs — no changes needed
hcie-egui-app/crates/hcie-gui-egui/src/canvas/render.rs — no changes needed
hcie-egui-app/crates/hcie-gui-egui/src/app/layout_solver.rs — no changes needed
hcie-egui-app/crates/hcie-gui-egui/src/event_bus.rs — no changes needed

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